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Availablesoon:
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VERYSHORTINTRODUCTIONS
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Astrobiology:AVeryShortIntroduction
ASTROBIOLOGY
AVeryShortIntroduction
DavidC.Catling
Contents
Acknowledgements
Listofillustrations
1Whatisastrobiology?
2Fromstardusttoplanets,theabodesforlife
3Originsoflifeandenvironment
4Fromslimetothesublime
5Life:agenome’swayofmakingmoreandfittergenomes
6LifeintheSolarSystem
7Far-offworlds,distantsuns
8Controversiesandprospects
Furtherreading
Index
Acknowledgements
IthankProfessorJohnArmstrong,DrRoryBarnes,ProfessorJohnBaross,Dr
BillyBrazelton,ProfessorRogerBuick,DrRachelHorak,ImeldaKirby,and
ProfessorWoodySullivanforreadingpartsofthemanuscriptandoffering
variouscorrectionsandsuggestions.ThanksalsotoLathaMenonandMimi
Southwood,whoreadtheentiremanuscript,thelatterofferingtheperspectiveof
anon-scientist.Numerousstudentswhoattendedmyastrobiologyclassesatthe
UniversityofWashingtonovertheyearsalsohelpedmementallypreparefor
writingthisbook.AtOxfordUniversityPress,IthankEmmaMaandLatha
Menonfortheirencouragementandassistance.
Listofillustrations
1TheHertzsprung–Russelldiagram
Adaptedfrom‘StellarEvolutionandSocialEvolution:AStudyinParallelProcesses’(2005),Social
Evolution&History.4:1,136–59),reproducedwithpermissionofProfessorRobertCarneiro
2Left:Cross-sectionoftheworld’soldestfossilstromatolitesRight:Aplan
viewofthebeddingplaneofthestromatolites
PhotographsbyDavidC.Catling
3Theapproximatehistoryofatmosphericoxygen
Authorsowndiagram
4a)Schematicofprokaryote(archaeaandbacteria)versuseukaryotestructure;
b)Twobacteriacaughtintheactofconjugation
b)Credit:CharlesC.BrintonJr.andJudithCarnahan
5Left:DNAconsistsoftwostrandsconnectedtogether.Right:Inthree
dimensions,eachstrandisahelix,sothatoverallwehavea‘doublehelix’
6Theclassificationschemeformetabolismsinterrestriallife
7The‘treeoflife’constructedfromribosomalRNA
8a)ValleynetworksonMars;b)OutflowchannelRaviVallis
a)ESA/DLR/FUBerlin
(G.Neukum);b)NASA/JPL/Caltech/ArizonaStateUniversity
9TheGalileanmoonsofJupiter:Io,Europa,Ganymede,andCallisto
NASA/JPL/DLR
10a)AnetworkofchannelsthatappeartoflowintoaplainneartheHuygens
landingsite;b)ImageofthesurfaceattheHuygenslandingsite.
CourtesyofESA/NASA/UniversityofArizona
11Brainandbodymassforsomedifferentmammals.
AdaptedfromComparativeBiochemistryandPhysiologyPartA:Molecular&IntegrativePhysiology,
Vol.136:4,Hassiotis,M.,Paxinos,G.,andAshwell,K.W.S.,‘Theanatomyofthecerebralcortexofthe
echidna(Tachyglossusaculeatus)’,827–50.Copyright(2003),withpermissionfromElsevier
Chapter1
Whatisastrobiology?
Behindthename
‘Whatthehellisastrobiology?’anAmericanSecretServiceagentcriedintohis
walkie-talkie.Hehadjustbeencheckingtheidentityofanacademicvisitorto
NASAsAmesResearchCenter,nearSanFrancisco.Thevisitorhadsaidthathe
wasattendingNASAsfirstastrobiologyscienceconference.Ameshasan
airstripthatprovidesasecurelandingsiteforAirForceOne,and,inApril2000,
PresidentBillClintonhadjustflownintovisittheSanFranciscoBayarea,
bringingalonghisSecretServiceentourage.
Theagent’squestionwasafairone.Itwasonlyinthelate1990sthatascientific
consensusemergedaboutthemeaningofastrobiology.FewlaymenorSecret
Serviceagentswouldhaveheardoftheterm.Backthen,NASAbeganto
promotearesearchprogrammeinastrobiologyledbyAmes,whereIwas
workingasaspacescientist.Atfirst,someofmycolleaguesdislikedtheliteral
Greekmeaningofthe‘biologyofstars’.Onenotedwithascoffhowlife
couldn’texistinsidetheinfernosofstars.Alesscurmudgeonlyinterpretationis
thatthe‘astro’inastrobiologyconcernslifearoundstars,includingtheSun,or
simplylifeinspace.Infact,manyastrobiologistsareasmuchconcernedwith
thehistoryoflifeonEarthaswithlifeelsewhere.Astrobiologistsagreethatwe
shouldhaveafirmunderstandingofhowlifeevolvedonEarthinorderto
pondertheexistenceoflifeinouterspace.Yetoneoftheastonishingaspectsof
modernscienceisthatithassofarfailedtoanswerquestionsaboutbiologythat
evenachildmightask.HowdidlifeonEarthgetstarted?Wehavesomeideas
butthedetailsareunknown.WhichspecialpropertiesoftheEarthandtheSolar
Systemmakeourplanethabitable?Again,somethoughtsbutthereisstillmuch
tolearn.Andwhatcausedlifetoevolveintocomplexorganismsinsteadof
remainingsimple?Again,we’reuncertain.
Tofilltheseholesinhumanknowledge,astrobiologyhasemergedasabranchof
scienceconcernedwiththestudyoftheoriginandevolutionoflifeonEarthand
thepossiblevarietyoflifeelsewhere.Thisismyownpreferreddefinition.NASA
hasdefinedastrobiologyasthestudyoftheorigins,evolution,distribution,and
futureoflifeintheuniverse.Othercommondefinitionsarethestudyoflifeinthe
universeorthestudyoflifeinacosmiccontext.Withinthispurview,
astrobiologistspursuethequestion‘What’sthehistoryandfutureofterrestrial
life?’aswellas‘Istherelifeelsewhere?’
Fourdevelopmentscoincidedwiththeemergenceofastrobiologyasadiscipline
inthelate1990s.In1996,controversialsignsofancientlifeweredescribed
withinaMartianmeteorite—a1.9kilogrampieceofrockthathadbeenblasted
offthesurfaceofMarsbyanasteroidimpactandhadeventuallylandedin
Antarctica.Whethertheinterpretationoffossilizedmicroscopiclifewascorrect
ornot(seeChapter6),itsetpeoplethinking.Furthermore,overthepreceding
twodecades,biologistshadestablishedthatsomemicrobesnotonlytolerateda
muchlargerrangeofenvironmentsthanhadpreviouslybeenthoughtbut
actuallythrivedinextremesoftemperature,acid,pressure,orsalinity.Soit
becameplausibletocontemplateextraterrestrialmicrobesexistinginseemingly
hostileplaces.Athirdfindingcamein1996frompicturestakenbyNASAs
Galileospacecraftoftheice-coveredsurfaceofJupitersmoon,Europa,which
revealedpiecesoficethathaddriftedapartinthepast,suggestinganocean
belowanicycrust.Then,fromthemid1990sonwards,astronomersfound
increasingnumbersofextrasolarplanetsorexoplanets,whichareplanets
orbitingnotourSunbutotherstars.Thepossibilitythatlifemightresideon
exoplanetsorinthecosmicbackyardofourownSolarSystemprovidedan
impetusforaskingwhetherlifemightbecommonintheuniverse.
Astrobiologyinthehistoryofideas
Althoughastrobiologycametotheforeinthe1990s,thequestionofwhether
we’realoneintheuniversegoesbackmillennia.Thales(c.600BC),often
regardedasthefatherofWesternphilosophy,espousedtheideaofapluralityof
worldswithlife.Subsequently,theGreekatomistschoolfromLeucippusto
DemocritusandEpicurus,whichbelievedthatmatterwasmadeofindivisible
atoms,favouredsuch‘pluralism’.Metrodorus(c.400BC),afollowerof
Democritus,wrote,‘Itisunnaturalinalargefieldtohaveonlyonestalkof
wheat,andintheinfiniteuniverseonlyonelivingworld’.Butitwouldbe
incorrecttoequatetheancientphilosophers’pluralismwithourmodern
conceptionoflifeonMarsorexoplanets.Metrodorushadnocluethatstarswere
Sun-likeobjectsatenormousdistancesandbelievedthattheyformeddailyfrom
moistureintheEarth’satmosphere.Thepopulatedworldsoftheatomists’
imaginationwerebodiesinanintangiblespace,similartomodernideasof
paralleluniverses.Inanycase,theopposingviewofPlato(427–347BC)and
Aristotle(384–322BC)ultimatelydominated.TheirbeliefthattheEarthwas
uniquelyinhabitedandatthecentreoftheuniverseprevailedforoverathousand
years.
Eventually,RenaissanceastronomersshowedthattheEarthorbitedtheSun.
WiththerealizationthattheEarthwasmerelyanotherplanet,speculationssoon
aroseaboutextraterrestriallifeonotherplanetsintheSolarSystem.Johannes
Kepler(1571–1630),theGermanastronomerresponsibleforastronomy’sthree
lawsofplanetarymotion,happilyentertainedtheideaofinhabitedplanets.Then,
bytheendofthe17thcentury,theDutchastronomerChristiaanHuygens(1629–
95)wasimagininglifebeyondtheSolarSysteminhisbookCosmotheoros
(1698):‘allthosePlanetsthatsurroundthatprodigiousnumberofSuns.They
musthavetheirplantsandanimals,nayandtheirrationalcreaturestoo.’
Extraterrestriallifewassoinvoguethatin1755thephilosopherImmanuelKant
(1724–1804)wroteofintellectualsonJupiterandamorousVenusians.
Incontrast,somescholarswithreligiousviewscontinuedtoclingtoEarth’s
uniqueness.AnexamplewasCambridgeUniversity’sWilliamWhewell(1794–
1866),whoseOfthePluralityofWorlds(1853)arguedagainstotherinhabited
planetsinasortofforerunnerofacontemporarydebatecalledtheRareEarth
HypothesisthatIdiscussinChapter8.
Bythelate19thcentury,theissueoflifeelsewherewasseenasapurely
scientificmatter,thoughscienceitselfsoondevelopedsomeblindalleys.
TelescopicobservationsbyGiovanniSchiaparelli(1835–1910)andPercival
Lowell(1855–1916)createdasurgeofinterestinthepossibilityofintelligent
lifeonMars.Unfortunately,Lowell’sbeliefthathesawcanalsonMarswasan
opticalillusioncreatedwhenthemindconnectsdotsinblurryimages,andhis
ideasofMartiancivilizationswerefantasy.Increasingly,asastronomers
employedpainstakingtechniquessuchasexaminingspectraoflightfrom
planets,itbecameapparentthatthephysicalconditionsonvariousSolarSystem
planetsmightnotbesofavourableforlifeafterall.Thependulumswungso
firmlyintheotherdirectionthatbythemid20thcenturyfewastronomerswere
interestedinplanets.IttooktheSpaceAgetorekindleoldcuriosities.
Whilethebasicquestionsofastrobiologyareancient,theterm‘astrobiology’
onlysurfacedfromtimetotimebeforebecomingcommoninthe1990s.In1941,
anessayentitled‘Astrobiology’byLaurenceLafleur(aphilosopheratBrooklyn
College,NewYork)describedthewordmorenarrowlythanitsmodern
incarnationastheconsiderationoflifeotherthanonEarth.OttoStruve,an
astronomerattheUniversityofCaliforniainBerkeley,alsousedthetermin
1955todescribethesearchforextraterrestriallife.ARussianastrophysicist,
GavriilTikov(1875–1960),andaGermanastronomer,JoachimHerrmann,
publishedbooksentitledAstrobiologyin1953and1974,respectively,covering
popularideasofextraterrestriallife.
Themodernuseof‘astrobiology’wasintroducedin1995byWesHuntress,then
atNASAsheadquartersinWashingtonDC.Atthetime,NASAscientistsargued
thatastudyoflifeoverscalesfromthemicrobialtothecosmicwasessentialfor
understandinglifeintheuniverse.Huntresslikedthewordastrobiologyforthis
aspirationandthenamestuck.
Infact,astrobiologywasreallyareinventionandexpansionofexobiology,a
fieldthatgoesbackseveraldecades.In1960,JoshuaLederberg(1925–2008)
coinedthewordexobiologyfor‘theevolutionoflifebeyondourownplanet’.
Lederberg,aNobelPrizewinnerfordiscoveriesinbacterialgenetics,arguedthat
anessentialpartofspaceexplorationshouldbesearchingforlife.Then,fromthe
1960sonwards,NASAfollowedLederberg’sadviceandfinancedexobiology
research.Butexobiologysoondevelopeditscritics.In1964,GeorgeGaylord
Simpson,aHarvardbiologist,quippedthat‘this“science”hasyetto
demonstratethatitssubjectmatterexists’.
Theadvantageoftoday’sastrobiologyisthatitcannotbeheldtoSimpson’s
chargebecauseitincludesthestudyoftheoriginandevolutionoflifeonEarth
atitscore.Astrobiologyalsoemphasizestheoriginandevolutionofplanetsasa
contextforlife,andsoembracesastronomersmorefirmlythanexobiology.
Exobiologyisnottheonlytermsimilartoastrobiology.Since1982,astronomers
haveofficiallyusedbioastronomyfortheastronomicalaspectsofthesearchfor
extraterrestriallife,whileearlierthewordcosmobiologywasfavouredbyJ.
DesmondBernal(1901–71),aninfluentialIrish-bornBritishphysicalchemist.
Butneitherofthosetermshasbecomewidespread.
Whatislife?
Astrobiologyraisesthedifficultquestionofhowtodefinelife.Whatisitexactly
thatwearelookingforbeyondEarth?Acommonapproachistolistlife’s
characteristics,whichincludereproduction,growth,energyutilizationthrough
metabolism,responsetotheenvironment,evolutionaryadaptation,andthe
orderedstructureofcellsandanatomy.Unfortunately,thiswayofdefininglifeis
unsatisfactoryforacoupleofreasons.First,thelistdescribeswhatlifedoes
ratherthanwhatlifeis.Second,mostoftheseaspectsoflifearenotunique.Life
hasstructuralordersuchascells,butsaltcrystalsarealsoordered.Someofmy
friendshavenochildrenbutthey’realive,Ithink,asaremulesthatcannot
reproduce.Growthanddevelopmentapplytolivingentities,butalsoto
spreadingfires.Alllifemetabolizesbutsodoesmycar.Lifereactstoits
environment,butamercurythermometeralsorespondstoitssurroundings.
Alternatively,somescientiststrytodefinelifeusingthermodynamics,thatis,
heatandenergyandtheirrelationshiptomatter,bysuggestingthattheessenceof
lifeisthepresenceofstablestructures,suchascellsandgeneticmaterial,
alongsideentropyproducedbymetabolicwasteandheat.
Thetermentropyrequiressomeclarification.Desperateteacherssearchingfora
quickanddirtyexplanationhaveoftencalledit‘disorder’.Entropyisnot
disorderbutanexactmeasureofenergydispersalamongstparticles,bethey
atomsormolecules.Energydispersesspatiallyandsotheenergyofgroupsof
particlesthatmovetogether,whichissaidtobecoherentenergy,candissipate.
Thus,abouncingballcomestorestbecauseitscoherentenergyofmotionis
convertedintoincoherentthermalmotionofmoleculesandatomsthrough
friction.Incontrast,astationaryballneverspontaneouslybeginstobounce(asif
itwerealive)becauseeventhoughsufficientthermalenergyexistsinthefloor
below,thatenergyisunavailableanddispersedinrandomjigglingoftheatoms
ofthefloor.TheSecondLawofThermodynamicsgovernssuchphenomenaand
statesthatentropyintheuniverseneverdecreases.Theentropyincrease(or
energydispersal)conservesenergybutruinsitsquality.High-qualityenergyis
notdistributedbutconcentrated,suchasinabarrelofoil,thenucleusofan
atom,orinphotons(particlesoflight)possessinghighfrequencyandshort
wavelength.Suchphotonsincludeultravioletandvisibleonesthatcausesunburn
andthatpowerplantlife,respectively.Inphysics,suchhigh-qualityenergyhas
lowentropy.
ThephysicistwhomostprominentlylinkedentropytolifewastheNobel
LaureateErwinSchrödinger(1887–1961).InWhatisLife?(1944),Schrödinger
commentedthatanorganism‘tendstoapproachthedangerousstateofmaximum
entropy,whichisdeath.Itcanonlykeepalooffromit,i.e.alive,bycontinually
drawingfromitsenvironmentnegativeentropy…Indeed,inthecaseofhigher
animalsweknowthekindoforderlinesstheyfeeduponwellenough,viz.the
extremelywell-orderedstateofmatterinmoreorlesscomplicatedorganic
compounds,whichservethemasfoodstuffs.Afterutilizingittheyreturnitina
verymuchdegradedform.’Regrettably,Schrödingerintroducedtheconceptof
‘negativeentropy’,whichdoesnotexistinscience,todescribetheordered
structureoffood.Also,inthegrowthofsomeorganisms,theincreaseinentropy
primarilycomesfromheatgenerationratherthanthedegradedformof
metabolicwasteproductscomparedtofood.LinusPauling(1901–94),whowas
arguablythegreatestchemistofthe20thcentury,bluntlyremarkedthat
Schrödinger‘didnotmakeanycontributionwhatever[toourunderstandingof
life]…perhaps,byhisdiscussionof“negativeentropy”inrelationtolife,he
madeanegativecontribution’.
Nonethelessacuriousby-productoftheever-increasingentropyintheuniverse
isthatordered,low-entropystructures,suchasorganisms,springintoexistence.
Infact,efficientproductionofentropyisbestachievedbyso-calleddissipative
structuresinvolvingacoherentstructureofanimmensenumberofmolecules
thatdissipatesenergy.Asimpleexampleisaconvectioncellinboilingwater.
Warmwaterrisesandisbalancedbysinkingwateronitsperiphery.This
circulatingcellhelpstodisperseenergyandsoincreasesentropymore
efficientlythanifthecellwereabsent.Alllivingorganismsarecomplicated
dissipativestructures.However,attemptstodefinelifewiththermodynamics
havesofarfailedtodistinguishclearlybetweenthelivingandnon-living.For
example,thewriterEricSchneiderdefineslifeasa‘farfromequilibrium
dissipativestructurethatmaintainsitslocalleveloforganizationattheexpense
ofproducingenvironmentalentropy’.Afirealsofitsthisdefinition.
Pauling’scriticismaside,Schrödingerarguedcorrectlythatorganismsmustruna
sortofcomputerprogram,whichiswhatwenowcallthegenome.Indeed,life
anywhereprobablyhastopossessagenome.Byagenome,wemeanaheritable
blueprintsubjecttosmallcopyingerrors,whichallowsanorganismtohave
evolvedfromanancestorandprovidesarecipeforlife’sothercharacteristics
suchasmetabolism.Evolution—thechangesinpopulationsoversuccessive
generationscausedbyselectionofindividuals’characteristics—isimportant
becauseitistheonlyprocessthatcanexplainthediversityoflifeandhowthe
featuresoflifethatwerelistedpreviouslywereconfigured.InDarwin’snatural
selectionmechanism,thegeneticvariationinpopulationsofindividualsmeans
thatsomearebetteradaptedforgreaterreproductivesuccessthanothers.Natural
selectionfavoursgenesthatleavemoredescendants,sothatlineagesaccumulate
geneticadaptations.
Mindfulofthecentralityofevolution,astrobiologistsoftendefinelifeas‘aself-
sustainingchemicalsystemcapableofDarwinianevolution’.Unfortunately,this
definitionisnothelpfulifwewanttodesignanexperimenttofindlife.Dowe
havetowaitforevolutiontohappenforapositivedetection?Abetterdefinition
usesthepasttense:‘lifeisaself-sustaining,genome-containingchemicalsystem
thathasdevelopeditscharacteristicsthroughevolution’.Sofar,space-bornelife
detectionexperimentshavenottriedtomeasureageneticmake-up.For
example,NASAsVikingLanderprobes,whichlookedforlifeonMarsinthe
1970s,weredesignedtorecognizetheEarth-likemetabolismofmicrobesinthe
soil(Chapter6).
ThephilosopherCarolClelandandscientistChristopherChybahavesuggested
thatattemptstodefinelifearelikethoseof17th-centuryscientiststryingto
definewater.Atthattime,waterwasconsideredacolourlessandodourless
liquidthatboilsandfreezesatcertaintemperatures.Withoutatomictheory,no
oneknewthatwaterisacollectionofmolecules,eachconsistingoftwoatomsof
hydrogenjoinedtoanoxygenatom.Byanalogy,perhapswelackthetheoryof
livingsystemsneededtodefinelife.
Manyoftheproblemsindefininglifeboildowntothefactthatwehaveonly
oneexample—lifeonEarth.AllEarth-basedorganismsusenucleicacidsfor
hereditaryinformation,proteinstocontrolbiochemicalreactionrates,and
identicalphosphorus-containingmoleculestostoreenergy.It’sthesamebasic
biochemistryinabacteriumorabluewhale.Soitisdifficulttodistinguish
whichpropertiesoflifeonEarthareuniqueandwhichareneededgenerallyto
qualifyas‘life’.Astrobiologycouldhelpsolvethisconundrumifwefoundlife
beyondEarth.
Thebarenecessitiesoflife
Whilethere’snoperfectdefinitionoflife,therearereasonablegroundstothink
thatcertainatomscommoninterrestrialbiochemistryarelikelytobeusedby
extraterrestrialorganismsandmighthelpusrecognizelifeelsewhere.OnEarth,
thechiefstructuralelementsinbiologyarecarbon,nitrogen,andhydrogen,
whilechemicalinteractionstakeplaceinliquidwater.Inastrobiology,there’s
wideagreementthatlifeelsewhereislikelytobecarbonbasedandthataplanet
withliquidwaterwould,atleast,favour‘lifeasweknowit.’Thesedeductions
arisefromrealizingthatlifeisconstructedfromalimitedtoolkit,theperiodic
tableofchemicalelements,whichisthesamethroughouttheuniverse.
Infact,carbonistheonlyelementcapableofforminglongcompoundsof
billionsofatomssuchasDNA(deoxyribonucleicacid).Consequently,only
carbon-basedextraterrestriallifeseemsabletohaveagenomeofcomparable
complexitytoterrestriallife.Carbonalsohasavarietyofotherpropertiesthat
allowauniquechemistryofitscompounds,sufficienttospawnthedisciplineof
organicchemistry.Carbon’sspecialpropertiesincludetheabilitytoformsingle,
double,andtriplebondswithitselfaswellasbondswithmanyotherelements.
Carboncanalsobuildthree-dimensionalcomplexitybyforminghexagonalrings
thatjointogether.
Becauselifehastogetstartedandpropagate,it’sprobablethatthemainatomsof
lifeareabundantones.Carbonisfourthincosmicabundanceafterhydrogen,
helium,andoxygen.Indeed,astronomershavefoundthatmanynon-biological
organicmoleculesalreadyexistinspace.Thesefreebiesmightserveas
precursorstolifegettingstarted(seeChapter3).Forexample,around30per
centbymassofthedustbetweenthestarsisorganicmaterial.So-called
carbonaceouschondritemeteoritesandinterplanetarydustparticlesinourown
SolarSystemcontainupto2percentand35percentorganiccarbonbymass,
respectively.
Becausesiliconhaschemicalpropertiessimilartocarbon,itissometimes
assertedthatsiliconmightallowanalternativeextraterrestrialbiochemistryto
carbon-basedmolecules,despitebeingabouttentimeslesscosmicallyabundant
thancarbon.Butinwater,atleast,siliconcompoundstendtobeunstableand
siliconeasilygetslockedintosolidsiliconoxides.Carbondioxideisagasat
commonplanetarytemperaturesanddissolvesinwatertoconcentrations
sufficientfororganismstousecarbondioxideasacarbonsource.Silicon
dioxide,incontrast,isaninsolublesolid,suchasquartz.Silicon’sbondswith
oxygenandhydrogenarestrong,whereascarbon–oxygenandcarbon–hydrogen
bondsaresimilarinstrengthtothecarbon–carbonbond,whichallowscarbon-
basedcompoundstoundergoreactionsofexchangeandmodification.Silicon–
hydrogenbondsalsotendtobeeasilyattackedinwater.Thestabilityofsilicon-
basedmoleculesrequireslowtemperaturestoslowdownreactionsthatwould
otherwisedestroythem.Appropriatelycoldsolventsincludeoceansofliquid
nitrogenonicyplanetsfarfromtheirstars.Atpresent,suchsilicon-basedlife
remainspurelyspeculative.
However,astablemediumisnecessaryforbiochemicalprocessessuchas
metabolismorgeneticreplication;onEarththismediumisliquidwater.For
extraterrestriallife,themediumcouldbeanotherliquidoradensegasaslongas
itdoesn’teasilybecomeasolidintheprevailingenvironment.Nonetheless,
water(H
2
O)hassomeuniqueproperties.Unlikeitssmellytwin,hydrogen
sulphide(H
2
S),whichcondensestoanastyliquidonlyat–61°C,waterturnsto
liquidbelow100°Catnormalpressures.Liquidwatersstabilityoccursbecause
theoxygenatominwatermoleculesisslightlynegativelychargedandallowsa
relativelystrong‘hydrogenbond’totheslightlypositivehydrogenatomsof
otherwatermolecules.Sulphurprovidesweakerhydrogenbondsbetween
hydrogensulphidemolecules.Wateralsoformsstrongerhydrogenbondsto
otherwatermoleculesthantomoleculesofoilysubstances.Asaconsequence,
oilsseparatefromwater,whichallowscellmembranestoformandprovide
homesforgenesandmetabolicprocesses.
Anotherunusualpropertyofwateristhaticeislessdensethanliquidwater.
Whenwaterfreezes,themoleculesalignintoring-likestructurescontaining
openholesontheatomicscale.Ificeweredenserthanliquid,thecoldbottomof
lakesandseaswouldcollectice,whichwouldbeinsulatedandremainfrozen.
Seaswouldfreezefromthebottomupandbecomeuninhabitable.Thiswould
arisebecausesunlightwouldbereflectedbackintheareaswheretheicereached
thesurface,causingcoolingandmoreicetoaccumulate.Gradualfreezingmight
betheunfortunatefateofseasofotherliquidssuchasammonia.Ammoniaisa
liquidfromabout–78°Ctoabout–33°Catapressureofoneatmosphere.But
anyseasofammoniawouldtendtosolidifyfromthebottomup,unlikeseasof
waterthatremainliquidevenifcoldtemperaturescauseanicecover.
OnEarth,wefindmicrobeswhereverthere’sliquidwater(excludingsterilized
apparatus),so‘lifeasweknowit’isasmuch‘waterbased’as‘carbonbased.’
Consequently,forSolarSystemexploration,thedetectionofliquidwaterorits
pastpresenceprovidesanobjectiveforplanetaryprobes,suchasthosevisiting
Mars.Nonetheless,itispossibletoconceiveoforganicsolventsasalternatives
towater,whichispotentiallyimportantforTitan,Saturn’slargestmoon(Chapter
6).
Anotherobservationfromterrestriallifeisthatjustsixnon-metallicelements—
carbon,hydrogen,nitrogen,oxygen,phosphorus,andsulphur—makeup99per
centoflivingmaterialbymass.Theseelementsareoftenabbreviatedas
‘CHNOPS’,butIprefer‘SPONCH’,whichiseasiertosay.WefindHandOin
water,whichmakesupmostlivingtissue,andC,H,andOinthenucleicacidsof
geneticmaterialandincarbohydrates.C,H,N,andSexistinproteins,whilePis
essentialfornucleicacidsandenergy-storagemolecules.Consequently,
detectingtheSPONCHelementsinchemicalformsthatlifecoulduseisanother
practicalgoalforplanetaryspaceprobessearchingforEarth-likelife.
Thesignificanceoflifeelsewhere
Youmightwonderwhetherthediscoveryofsimple,microbial-like
extraterrestrialswouldreallymatter.Butifwecouldfindasingleinstanceoflife
thatoriginatedelsewhere,itwouldprovethatlifeisnotamiracleconfinedto
Earth.Wewouldn’tbealone.EventhesimplestmicrobesnativetoMarsorto
Europa’soceanswouldchangethebalanceofprobabilitiesthatlifeexists
elsewhereinthegalaxyfortheywoulddemonstratethatlifecanoriginatetwice
withinonesolarsystem.Atthemoment,wehavenoconvincingevidenceoflife
beyondEarth.However,inChapter6,Iwillarguethatatleastnineotherbodies
inourSolarSystemmightbehabitabletoday,ifwekeepanopenmind.Solar
Systemastrobiologyisfarfromsettled.
Asecondsignificantfactorinfindinglifeharksbacktothedifficultiesof
defininglife.TheplanetaryscientistCarlSagan(1934–96)commentedinhis
bookCosmicConnection(1973)that‘thesciencethathasbyfarthemosttogain
fromplanetaryexplorationisbiology’.Anexaminationofextraterrestriallife
wouldbeofprofoundsignificancenotonlyinidentifyingthoseelusive
characteristicscommontoalllifebutalsoinsheddinglightonhowlife
originated,whichremainsunsolved.
Chapter2
Fromstardusttoplanets,theabodesforlife
Tomakeanapplepiefromscratch,youmustfirstinventtheuniverse.
CarlSagan(1980)
Whentheuniversebegan,temperatureseverywherewerefartoohotforatomsto
bestable,letalonejoinupintocomplexbiologicalmolecules.Lifeexists
becausefollowingaBigBang13.8billionyearsago,ahot,densecosmos
expandedandcooled.Asitdidso,atoms,galaxies,stars,planets,andlifearose.
Here,weexaminehowthisprocessproducedanabodeforlife—theEarth.
Westartwiththestructureofthepresentuniverse,whichprovidestheclues
aboutitshistory.Imagineajourneythatgoestotheedgeoftheobservable
universe.Atthespeedoflight—300,000kmpersecond—itwouldtakeonly1.3
secondstogettotheMoonatitsdistanceof384,000km.AdiagramoftheEarth
representedasaspotof2.7mmdiameterandtheMoonlooksasfollows:
Thisscaleisfairlyeasytograsp.Butitchallengesourimaginationwhenwe
realizethattheSunonthesamescalewouldbe30cmindiameterandwewould
havetoplaceitjustover30mawayfromthisbook.Theneareststar,Proxima
Centauri,whichissmallcomparedtotheSun,wouldbeabout4cmindiameter.
Tokeeptoscale,wewouldhavetoplaceit8,650kmaway,whichisroughlythe
flightpathfromSanFranciscotoLondon.
Continuingourvoyageatthespeedoflight,itwouldtake8.3minutestofly
fromtheEarthtotheSunandalittleoverfourhourstothentraveltotheaverage
orbitaldistanceofNeptune,theoutermostoftheeightplanets.After4.2years,
wewouldreachProximaCentauri.Withthehugedistancesinvolved,wedefine
thedistancetravelledinayearatthespeedoflight,some9,500billionkm,asa
lightyear,sothatProximaCentauriis4.2lightyearsaway.
TheSunandProximaCentauriaretwoofabout300billionstarsintheMilky
Waygalaxy,whichisadiscsome100,000lightyearsacrosswithstars
concentratedinspiralarms.Galaxiescontainmillionstotrillionsofstars,sothe
MilkyWayismoderatelylarge.TheSolarSystemsitsintheOrionArm,two-
thirdsoutfromtheGalacticCentre.Thisarmappearsunremarkablecomparedto
someothersthataremorerichlypopulatedwithstars.Butit’spossiblethatthe
SolarSystem’slocationinthegalacticstickswasactuallyvitalforterrestriallife.
TheEarthmayhaveavoidedcertaincatastrophes,suchasproximityto
explodingstars.Ifso,theremaybeaparticularregioningalaxiesfavourablefor
life,calledthe‘GalacticHabitableZone’,discussedinChapter7.
Onalargerscale,therearemorethan100billiongalaxiesintheobservable
universearrangedintogroupsandsuperclusters.Withinadiameterofabout10
millionlightyearscentredbetweentheMilkyWayandournearestspiralgalaxy,
theAndromedagalaxysome2.5millionlightyearsaway,thereareroughlyfifty
galaxies,formingtheLocalGroup.Inturn,thisgroupisoneofahundredorso
withinasphereof110millionlightyears’diameter,comprisingtheVirgo
Supercluster.Mapsthatcoverbillionsoflightyearsshowfilamentsoftiny
scatteredpointswhereeachpointisagalaxy.Inthreedimensions,galactic
filaments,whicharethelargeststructuresknowntohumankind,joinupintoa
webseparatedbyvastvoids.Thewholefantasticstructurelooksasifitwere
spunbyacrazyintergalacticspider.
Howbigistheobservableuniverse?Ifspacehadnotexpanded,thefarthest
distancewouldbe13.8billionlightyears,whichisthattraversedbyaphoton—a
particleoflight—sincetheBigBanghappened13.8billionyearsago.Butspace
hasexpanded.Sotheactualsizeoftheobservableuniverseisnowabout47
billionlightyearsacross.Suchvastnessisaconsiderationforastrobiology
becausesurveysofplanetsaroundstarsjustwithintheMilkyWaysuggestthat
eachstarhostsatleastoneplanetonaverage.Somefractionofplanetsoughtbe
habitable,perhapsatleast1percent,sothenumberofpotentialabodesforlife
mightexceedatrillionbillion.
ThestructureoftheuniversetracesbacktotheBigBang.Inthe1920s,
telescopicobservationsbytheAmericanastronomerEdwinHubbleshowedthat
galaxiesaremovingawayfromeachotheronthelargescaleasspaceexpands
betweenthem.Thus,goingbackfarintime,everythingmusthavebeen
scrunchedupandveryhot.Thisconsiderationledtotherecognitionofstrong
evidencefortheBigBang.Itsafterglow,theCosmicMicrowaveBackground,
permeatestheentireuniverse.IftheBigBangistrue,physicsdictatesthatbefore
theearlyuniversecooled,itmusthavebeenanopaquefireballmadeofelectrons
andprotons(theelectricallynegativeandpositiveelementaryparticlesthatmake
upatoms),photons(particlesoflight),andsomesmallgroupsoffusedprotons.
Some380,000yearsaftertheBigBang,itbecamecoolenoughthatelectrons
wereabletojoinwithprotons,orgroupsofprotons,toformthetwosmallest
atoms,hydrogenandhelium.Atthatpoint,theuniversebecametransparentto
lightbecausepreviouslyphotonshadbeenscatteredbythefree-floating
electrons.Thispriorsituationwasanalogoustothewaythatlightbounces
aroundofftinydropletsinafogsothatyoucan’tseethrough.Sincelosingits
opaqueness,theuniversehasexpandedbyaboutafactorof1,000,andthe
wavelengthoftherelicphotonsfromtheBigBanghasbeenstretchedthesame
amount,changingthephotonsfromredlighttomicrowave.Amazingly,when
youtuneanoldanaloguetelevisionorradiobetweenchannels,theleftover
radiationfromtheBigBangcontributestothestatichiss,albeitatalevelof
about1percentorless.Infact,in1964,thisnoiseinalargeradiohornwashow
themicrowavebackgroundwasdiscovered.Pigeondroppingsinthehornwere
blamedinitially;butaftercleaningupandshootingtheunfortunatepigeons,the
realculpritwasidentifiedasthebeginningoftheuniverse.
GalaxiesappearedafewhundredmillionyearsaftertheBigBang.Someplaces
hadveryslightlymorematerialthanaverageandsohadhighergravity.
Clumpingproducedgalaxies,andwithinthegalaxies,onasmallerscale,gas
cloudscollapsedundertheirowngravity.Theinteriorofeachshrinkingcloud
heatedupasgasparticlescollided,eventuallymakingahot,glowingballofgas
—astar.
Partofthe‘astro’inastrobiologycomesfromthefactthatalloftheatomsused
bylifeexceptforhydrogenwerecreatedinsidestars.Thefirststarswouldhave
beenmadeonlyoftheelementssynthesizedintheBigBang:hydrogen,
comprisingthree-quartersofthemass,andhelium,whichmadeuptherest
exceptforatraceoflithium.Theoxygeninwater,thenitrogeninproteins,orthe
carbonineveryorganicmolecule—noneoftheseelementswaspresentatfirst.
Buteventuallystarsmadethem.
Tounderstandhowstarsmakeelements,considerhowtheSunshines.Insidethe
Sun,immenseheatstripseachatomdownintoitsconstituents:apositively
chargednucleusandnegativelychargedelectrons.Thetemperatureatthecentre
oftheSun,some16milliondegreesCelsius,isenoughtofusethenucleioffour
hydrogenatomsintoaheliumnucleus,whichisanuclearreactionthatreleases
photons.Eachphotonthenenduresaone-million-yearjourneyfromtheinterior
oftheSuntospace.Ittakessolongbecauseeachphotoniscontinuallyabsorbed
andemittedasitencountersmaterial.Thephotonalsolosesenergy.Itstartsout
asahigh-energygammarayandonaverageturnsintoalower-energyphotonof
visiblelightbythetimeitescapesfromtheSun.Inthe1950s,physicists
reproducedthekindofnuclearreactionsthatoccurinsidetheSunwithhydrogen
bombs.CoresofstarslikeourSunareakintohydrogenbombsthat,ineffect,
can’texplodebecausetheyarecontainedbytheweightofmaterialabovethem.
TheSunwillnotfusehydrogeninitscoreforeverandtherepercussionswill
destroylifeonEarth(partlyansweringastrobiology’squestionof‘What’sthe
futureoflife?’).Inmoststellarcores,theaccumulationofhelium‘ash’causes
temperaturestodroptoolowtosupportfurtherhydrogenfusion.Atthispoint,
thestarshrinksunderitsownweight,whichcausesthetemperaturetoriseuntil
itigniteshydrogenfusioninashellsurroundingthecore.Theenergyrelease
causesouterlayersofthestartoexpand,cool,andredden.Thisishowared
giantforms,suchasAldebaran,thebrighteststarintheconstellationofTaurus,
theBull.TheSunwilleventuallybecomearedgiantandswelltwo-hundredfold
by7.5billionyears’time,probablyengulfingtheEarth.Theweightoffurther
heliumasheventuallysqueezesaredgiant’scoretoatemperatureof100–200
milliondegreesCelsius,whichisenoughtofuseheliumnucleiandmakecarbon
andoxygen.Inturn,ahelium-burningshellcaneventuallysurroundacoreof
carbonandoxygen‘ash’.StarswithfourtoeighttimesthemassoftheSuneven
endupfusingthecarbonandoxygenintoheavierelements,includingneonand
magnesium.
Generally,thedeaththroesofaSun-likestarinvolvesheddingouterlayersinto
space.Theseshellsofglowinggasarecalledplanetarynebulaebecausethey
looksomewhatlikeplanetsthroughlow-magnificationtelescopes,buttheyhave
nothingtodowithplanets.Theremnantsofthestarcooldownintoawhite
dwarfwitharadiuscomparabletothatoftheEarthbutanenormousdensity.In
theory,awhitedwarfstopsshiningaftertenstohundredsofbillionsofyears,
producingablackdwarf.Buttheuniverseisnotyetthatold.
StarslargerthanabouteighttimesthemassoftheSuneventuallyexplodeas
supernovae.TheSunisabouthalfwaythroughitsten-billion-yearphaseof
hydrogenfusioninitscore,butthesemassivestarsspendlessthan60million
yearsinthesamephasebeforebecomingredsupergiants,ofwhichBetelgeuse,
intheconstellationofOrion,isanexample.Insuchstars,nuclearfusionin
shellsaroundthecoreproducestheelementsneon,magnesium,silicon,andiron.
Ironisgenerallytheheaviestelementmade,althoughsomeheavierelementsare
alsoproducedwhenfreeneutronsareaddedtoexistingnuclei.(Neutronsare
particlesthathavenoelectricalchargeandarecommonlyfoundinatomic
nuclei.)Whenthefuelinsuchastarrunsout,thecoreissocompressedthat
negativelychargedelectronsamalgamatewiththepositivelychargedprotonsof
theironnuclei.Theelectricalchargescanceloutandunchargedneutronparticles
arecreated.Asaresult,thestellarcoreshrivelstoabout12kmdiameter,
formingsomethinglikeagiganticatomicnucleusmadeonlyofneutrons.
Becauseoftheshrinkage,therestofthestarcollapsesontothedenseneutron
core,andaviolentreboundcreatestheincrediblybrightsupernova.
Elementsheavierthanironaregeneratedanddistributedbysupernovae.Afew
secondsafterasupernova,theouterlayersofthestarareheatedtoanincredible
10billiondegreesCelsiuswhilethebreak-upofnucleifromdeeperlayers
suppliesabundantneutronstofuelreactionsthatmakeheavyelements.This
cosmicalchemycreatestheheavy,preciouselementssuchasgold,silverand
platinum.Importantly,thematerialblownoutinasupernovaformsthebasisof
newgenerationsofstarsandabodesforlife,theirplanets.Inthecaseofstars
reachingtensofsolarmasses,althoughasupernovastilloccurs,thecollapseat
thecentreproducesablackhole—anobjectsomassivethatnothingescapesits
gravity,includinglight.
1.TheHertzsprung–RusselldiagramfortheSunandnearbystars.
TemperaturesareinKelvin,whichis273plusthetemperatureindegrees
Celsius;theKelvinscaleisdefinedsothat0Kor‘absolutezero’iswhereall
molecularmovementstops
Thephasewhenastarconvertshydrogentoheliuminitscoreisthemain
sequencelifetime,whichisgenerallytheintervalweconsideroptimalforlifeto
thriveonplanetsaroundthestar.The‘mainsequence’referstoadiagonal
swathefromupperlefttolowerrightonthemostfamousgraphinastronomy,
theHertzsprung–Russell(H–R)diagram(Fig.1),namedafteritstwo
originators.Thegraphplotsastarsluminosityversusitssurfacetemperature.
Bya‘surface’,astronomersdon’tmeanahardsurfacebutthelevelinastars
atmospherewheremostlightemerges,whichisasdeepaswecansee.
Oddly,thetemperatureaxisrunsbackwardsfromhightolowintheH–R
diagram.Thepurposeistomatchthecolourcodingofstarsfromhotblue-white
starstocoolerredoneswiththelettersO,B,A,F,G,K,andM.Thelettergives
astarsspectralclass.Generationsofastronomystudentshaveremembered
spectraltypeswiththemnemonic‘OhBeAFineGirl/GuyKissMe!’(theletters
don’tstandforanythingandhaveoriginsin19th-centuryastronomy,whichneed
notconcernus).
Onthemainsequence,themostmassivestarsplotattheupperleftandthe
lightestatthelowerright.Whateveritsmass,amainsequencestariscalleda
dwarf—suchastheSun,aG-typedwarf.Thelifetimeonthemainsequencecan
beover50billionyearsforcoolreddwarfs.
Theimportancetoastrobiologyofthewaystars‘live’and‘die’iswideranging.
OurSunisamiddle-aged,mainsequencestarwithstablesunlightthatfuels
mostlifeonEarththroughphotosynthesis.Asmentionedattheoutset,theatoms
oflifeweregeneratedinredgiantsandsupergiants.Also,oxygen,silicon,
magnesium,andironaremadefromintegralnumbersofheliumnucleiandso
areparticularlyabundantproductsofnuclearfusion,whichissignificantbecause
theseatomsaretheonesthatmakerocks.Rockyplanets,liketheoneweinhabit,
areanaturalconsequenceofthephysicsofstarlight.WealsoknowthattheSun
isatleastasecond-generationstarbecausewehavesupernovaelementsonEarth
suchasgold.ButsincetheSunisonly4.6billionyearsoldina13.2billion-
year-oldMilkyWay,manystarscameandwentbeforelifearoseonEarth.Did
earlierstarssupportplanetsandlife,orevenintelligentlife,andwhathappened
tothem?Thisleadsustothequestionofhowourownplanetformed.
Gettingaplacetolive:whereplanetscomefrom
IdeasfortheoriginoftheSolarSystemhavealongheritage.In1755,Immanuel
KantsuggestedthattheSolarSystemcoalescedoutofadiffusecloudinspace.
Later,in1796,themathematicianPierreSimon,theMarquisdeLaplace,
elaboratedthenotion.ThebasicKant–Laplaceconceptisknownasthe‘nebular
hypothesis,’afternebula,theGreekwordforcloud.
Thehypothesisstartswiththeideathatsomepartofthecloudisslightlydenser
thanothersandattractsmaterialbygravity.Itislikelythatthecloudalsohas
someslightinitialrotation,sothattheshrinkingcloudspinsfaster,likeanice
skaterdrawinginherarms.Therandommotionofthegasanddustwilloppose
materialattractedalongtheaxisofrotationbutmatterconvergingintheplaneof
rotationwillalsoberesistedbythespin.Asaresult,thecloudflattensintoa
disc.TheSunformsinthecentre,whileplanetscoalesceintheplaneofthedisc
fromsparsematerial.Thisisconsistentwiththemassoftheplanetsbeingonly
0.1percentoftheSun’smass.Becausetheyformfromadisc,planetswillallbe
inthesameplaneandtheywillallorbittheSuninthesamedirection,aswe
observe.
Inrecentdecades,itwasthoughtthatevidencefromisotopessuggestedthata
shockwavefromanearbysupernovatriggeredthecollapseofthenebula.
Isotopesareatomsthatcontainthesamenumberofprotonsintheirnucleusbuta
differentnumberofneutrons.TheGreekroot,isostopos,means‘equalplace’,
whichreferstothesamelocationintheperiodictableofelements.Sometimesan
isotopehasanucleusthatistoobigtobestable,andbreaksapartbyradioactive
decay.Anunstablealuminium-26atom(whichhasanucleusof26particles=13
protons+13neutrons)decaysintostablemagnesium-26(containing12protons
+14neutrons).Inanysampleofaluminium-26atoms,thetimeittakesforhalf
ofthemtochangeintomagnesium-26,thehalf-life,is700,000years.The
presenceofmagnesium-26insomemeteorites,whichmusthavebeenproduced
relativelyquicklyfromaluminium-26,wasthoughttosuggestanearby
supernovawhentheSolarSystemformedbecausemassivestarsmake
aluminium-26andtheirsupernovaedistributeit.
However,in2012,newmeasurementsinmeteoritesshowedthatthelevelsof
iron-60,whichisanisotopeonlyformedinsupernovae,aretoolowforanearby
supernova.Toexplainabundantaluminum-26,aneighbouringmassivestar
(perhapsmorethantwentytimesthesolarmass)ofatypeknownasaWolf–
Rayetstarmayhavesheditsouterlayersandspreadaluminum-26intothesolar
nebula.Aluminum-26wasadominantheatsourceinthesolarnebulaforthefirst
fewmillionyears.Bymeltingiceintheearliestrockymaterial,thealuminum-26
causedwatertogointohydratedmineralswhereitwassafe,unlikeicethat
evaporates.Ifthealuminum-26hadnotbeenpresent,theEarthmightnothave
gainedwater-richmineralsanditmighthaveneitheroceansnorlife.
Thenebularhypothesisexplainsthebroaddistributionofdifferenttypesof
planet.TheplanetsintheinnerSolarSystem(Mercury,Venus,Earth,andMars)
arerelativelysmallandrocky,whilethoseintheouterSolarSystem(Jupiter,
Saturn,Uranus,andNeptune)aregiants.Whenthediscwasforming,thematter
drawninwardsgainedenergyofmotionandthecentreofthedisc,wheretheSun
formed,becameextremelyhot.Thiscreatedatemperaturegradientfromthehot
centreofthedisctoacoldexterior.OutsidetheSun,pressureswerelow,which
meantthatsubstancesexistedeitherassolidsorgasesbutnotliquids.Inthe
innerdisc,itwasfartoohotforgasessuchaswatervapourtoformice,but
metalandrockvapourcouldcondensenearlyanywhereinthedisc.
Consequently,planetsthatformedintheinnerregion,suchastheEarth,ended
upwithiron-richcoressurroundedbyrockymantles.Incontrast,water
condensedintoicefromjustinsidetheorbitofJupiter(‘theiceline’)andfarther
out,providingmorematerialtomakelargerplanets.Methanecouldalso
condenseasanicefromtheorbitofNeptuneoutwards.
Thegiantplanetsareconsideredtohaveformedbeforetherockyones.Jupiter
andSaturnprobablyformedwhenrockycoresreachedasizeofabouttenorso
Earthmassesthathadsufficientgravitytoattractmoreandmorehydrogenand
heliumgasdirectlyfromthediscuntilallthatwasavailableintheirorbitwas
suckedup.Becausetheyarehugeballsofmostlygas,wecalltheseplanetsgas
giants.Thisprocesshappenedwithinabout10millionyearsaftertheformation
oftheSun.UranusandNeptunearesmaller,anddrewinagreaterproportionof
icysolids,sowecalltheseplanetsicegiants.UnliketherapidgrowthofJupiter,
theformationoftheinnerrockyplanetswasspreadover100–200millionyears.
Planetesimals,whichare‘piecesofplanet’,coalescedtomakelarger,rocky
objectscalledplanetaryembryoswithasizebetweenthatoftheMoonandMars.
Inturn,severalplanetaryembryosmergedintoVenusandEarthandfewerinto
MercuryandMars.
Althoughtheinnerplanetsaccumulatedmaterialintheirlocality,gravitational
nudgesfromthegiantplanets,particularlyJupiter,wouldhavesent
planetesimalscareeringintotheinnerSolarSystem.Scatteredhydratedasteroids
thatoriginatedfrombeyondtheorbitofMarswerelikelyresponsiblefor
bringingwatertotheEarththateventuallyturnedintoouroceansandlakes.We
drinkasteroidwater.ComputersimulationsshowthatJupiterejectedmore
water-richmaterialthanitscatteredinward,sothatifJupiterhadhadaless
circularorbitandejectedevenmorewater-richmaterial,Earthmighthaveended
upwithoutoceansandlife.
Wealsoknowthatplanetsdonotnecessarilystayputintheirorbits.So-called
hotJupitersareexoplanetssimilarinmasstoJupiterthatorbitatleasttwiceas
closetotheirhoststarsastheEarthorbitstheSun.Suchexoplanetscannothave
formedwheretheycurrentlyresidebecauseitwouldhavebeentoohot.Itturns
outthatplanetsmigratebecauseoflargetailsofgasanddustinthenebulathat
accompanytheirformationaswellasthegravitationalinfluencefromother
bodies.Sothetraditionalnebulahypothesisisnuancedbyplanetarymigration
thatcoulddestroyorfavourplanetaryhabitabilityinextrasolarsystems,
dependingonthedetails.Inthenextchapter,I’lldiscusstheideathateventhe
giantplanetsofourownSolarSystemmayhavemigratedsomewhat.
Nonetheless,theessentialideaofthenebularhypothesis—thattheplanets
formedfromadisc—wasconfirmedinthe1980swhendiscsofdebrisaround
youngstarswereseenthroughtelescopes.Infact,thereisstillsomeleftover
debrisinourownSolarSystem.Cometsareicybodiesandasteroidsarerocky
rubblethatwereneverassimilated.Occasionally,smallchunksthathavebeen
knockedoffasteroidsthroughcollisionsendupfallingontotheEarth’ssurface
asmeteorites.Consequently,meteoritesprovideuswithkeydataabouttheearly
SolarSystem.
TheageoftheEarthandtheMoon
Perhapsthemostprofoundinformationgleanedfrommeteoritesistheageofthe
EarthandSolarSystem.Eversincethe18thcentury,thevastlayersof
sedimentaryrockseenbygeologistshadledthemtosuspectthattheEarth,and
hencetheSolarSystem,mustbeofgreatage,butproofwaslacking.
ThefirstpersontoattempttomeasuretheageoftheEarthwasanEnglish
geologist,ArthurHolmes,whohadtheideaofexaminingleadisotopesformed
fromradioactiveuranium.Uranium-238decaysintoacascadeoffurther
unstableisotopesofotherelementsuntilreachingastableleadisotope,lead-206.
Thehalf-lifeforuranium-238tochangeintolead-206is4.47billionyears.
Anotherradioactiveisotope,uranium-235,decaysintolead-207withahalf-life
of704millionyears.So,bymeasuringtheamountsoflead-207andlead-206in
differentmineralgrainsofarock,youcandeterminetheageoftherock,because
althoughtheremayhavebeendifferentamountsofuraniumineachgrain,the
fixeddecayratesaddthesameratiooflead-206tolead-207inagiventime.In
1947,HolmesappliedhismethodtoapieceofleadorefromGreenland,and
estimatedthattheEarthformedby3.4Ga(whereGameans‘Gigaanna’,
equivalentto1,000millionyearsor‘billionsofyearsago’).
Holmeshadtwoproblems.First,hecouldneverbesurethateventheoldestrock
hecouldfindwasasoldastheEarthitself.Second,forapreciseage,Holmes
neededtoknowthesmall,originalratioofleadisotopespresentwhentheEarth
formed,so-called‘primevallead’,beforesubsequenturaniumdecayadded
furtherleadatoms.ClairPatterson,anAmericangeochemist,realizedthatthe
firstproblemcouldbeavoidedbylookingatmeteoritesbecausetheseare
leftoverbuildingmaterialsthatformedaroundthesametimeastheEarth.He
alsorecognizedthatcertaintypesofmeteorite,theironmeteorites,contain
negligibleuraniumandsotheirratioofleadisotopesprovidesameasureof
primevallead.Withthisapproach,in1953,PattersonaccuratelyagedtheEarth
at4.5Ga.Hewassoexcitedthathefearedaheartattack,andhismotherhadto
takehimtohospital.
Sincethen,improvedtechniquesusingradioactiveisotopeshavegivenusa
timelinefortheeventssurroundingtheEarth’sformation.Theveryoldestgrains
inmeteoritessuggestthattheSolarSystemformedat4.57Ga.TheEarthformed
slightlylaterat4.54Ga.Then,around4.5Ga,theEarthwasapparentlyhitbya
Mars-sizedobject,whichisnamedTheiaaftertheGreekgoddesswhogavebirth
totheMoongoddessSelene.Accordingtothisgiantimpacthypothesis,debris
thatwasblastedoutfromtheimpactwentintoorbitaroundtheEarthand
coalescedtoformtheMoon.
TheMoonisimportantforastrobiologybecauseitsgravitystabilizesthetiltof
Earth’saxistotheplaneofitsorbit,whichhelpstheEarthmaintainarelatively
steadyclimate.Currently,theEarth’saxisistilted23.5degrees,butifitwere
abletovarywidely,greatclimaticswingswouldoccur.Forexample,at90
degreestilt,theEarthwouldbetippedonitssideandicewouldformseasonally
attheequator.ComputersimulationsshowthatifwehadnoMoon,theEarth’s
axialtiltwouldvarychaoticallyoverperiodsofmillionsofyearsandhavea
largerange,potentiallyfrom0tomorethan50degrees.Microbiallifecould
probablywithstandlargeclimateswings.Butadvancedanimallifeand
civilizationssuchasourswouldbechallenged.
Infollowingourastronomicaltrail,wehavenowarrivedatthepointofthe
creationofanabodeforlife,ourownplanet.Therewerecontingenciesin
whetherthewaternecessaryforlifewasdeliveredtotheEarthandcontrolson
whereanEarth-likeplanetmightoccur.Then,afterEarthformed,howandwhen
didlifearise?
Chapter3
Originsoflifeandenvironment
TheearlyEarth
SolittleisknownabouttheveryearliestaeonofEarthhistorythatitdoesn’t
evenhaveanofficialname.Informally,itiscalledtheHadean,whichstarted
around4.5GawhentheMoonformed,andendedatadatethathasn’tbeen
agreeduponbutisusuallytakenaseither3.8or4.0Ga.Itisprobablethatlife
originatedintheHadean,butsofarwehaven’tfoundevidencebecausethereare
nosedimentaryrocksfromthistime.Suchrocksconsistoflayersofsediment
grainslaiddowninwaterorfromairfall,andsobestpreservetracesofbiology
ortheenvironment.Consequently,ourideasofwhathappenedduringthe
Hadeanhavetobeconstructedfromsparsedataaidedbythetheoretical
constraints.
TheorysuggeststhatheatfromthegiantimpactthatformedtheMoonwould
haveturnedrockintogas.Theatmosphereofvaporizedrockwouldhavelasted
forafewthousandyearsandthencondensedandraineddownonamolten
magmasurface,whicheventuallysolidifiedintoacrust.Subsequently,the
atmospherewouldhaveconsistedmainlyofextremelydensesteamforafew
millionyearsbeforeitcondensedtoformoceans.
Whendidcontinentsbegintoform?Wegetsomecluesfromtinymineralgrains
lessthan0.5mmacrossthatareleftbehindfromthefirsthalfbillionyearsorso
ofEarthhistory.Thesearezircons,crystalsofzirconiumsilicatewithchemical
formulaZrSiO
4
,whereZriszirconium,Siissilicon,andOisoxygen.Zircons
aresotoughthattheyremainevenaftertherockinwhichtheywereoncehosted
haserodedaway.Somezirconsasoldas4.4to4.0Gahavebeenfoundin
fossilizedgravelintheJackHills,whichareaboutathousandkilometresnorth
ofPerthinwesternAustralia.Thezirconscontainflecksofquartz,whichisa
crystallineformofsilica,SiO
2
.Thequartzmayhavebeenderivedfromgranites,
whichisthesilica-richtypeofigneousrockthatmakesupmuchofthe
continents.Inthisway,zirconssuggestthatEarth’scontinentalcrustexistedas
longagoas4.3Ga.Isotopessupportthisinference.Someancientzirconsare
enrichedinstableoxygen-18atomsrelativetostableoxygen-16ones.Such
enrichmentoccurswhensurfacewatersmakeclaysandthemudisthenburied
andmeltsunderground,passingtheisotopicsignaturetoigneousrocks.
IntheHadean,theEarthwasprobablyhitbyafewhugepiecesofdebrisleft
overfromSolarSystemformation,butnoneasbigastheMoon-forming
impactor.Theenergyofasmallnumberofverylargeimpactscouldhave
vaporizedtheentireoceanoritsupperfewhundredmetres.Ifso,lifewould
havehadtorestartoritmighthavebeentrimmedbacktoonlythosemicrobes
shelteredundergroundthatwereabletosurvivetheheat.Infact,partially
sterilizingimpactsmayexplainthenatureofthelastcommonancestorforall
lifeonEarth.Geneticstracesthecommonancestortoathermophile(seeChapter
5)—amicrobethatlivesinhotenvironments.Essentially,DNAanalysisimplies
thatyour‘great-great-great-…grandmotherwasathermophile,ifyouinsert
enough‘greats’.Thismaybebecausethermophilesweretheonlysurvivorsof
hugeimpacts.
TheEarthtodayisnotentirelyfreefrompotentiallycatastrophicimpacts.For
example,Chironisacomet-likeobjectabout230kmacrossintheouterSolar
SystemthatcrossestheorbitofSaturn.Onedaywithinthenext10millionyears
orso,anudgefromSaturn’sgravitywillflingChirontowardstheSunoraway
fromit.Intheformercase,Chiron’schancesofimpactingtheEarthwouldbe
lessthanoneinamillion.ButifChirondidhit,theheatwouldturntheupper
fewhundredmetresoftheoceanintosteam,andlandwouldbesterilizeddown
toaboutfiftymetres’depth.Thermophilesmightbecometheancestorsof
subsequentlifeinasortofevolutionarydéjàvu,iftheimpactexplanationfora
thermophileancestoriscorrect.
ThelasthurrahofthebigHadeanimpactsisknownastheLateHeavy
Bombardment,whichhappenedabout4to3.8Ga.CratersontheMoonbear
witnesstothismassivebombardment.RocksbroughtbackbyApolloastronauts
havebeendatedusingradioisotopesandindicatethatmanycraterswere
producedwithinthesame200-million-yearinterval.
TheleadinghypothesisforexplainingtheLateHeavyBombardmentwas
devisedbyastronomersinNice,France,andisthereforecalledtheNicemodel.
ItreliesontheastonishingideathattheorbitsofJupiterandtheothergiant
planetsshiftedattheendoftheHadean.CalculationssuggestthataftertheSolar
Systemformed,mutualgravitationaleffectscausedSaturnandJupitertoreacha
statecalleda‘resonance’inwhichSaturnorbitedtheSunonceforeverytwo
Jupiterorbits.Regularalignmentcreatedperiodicgravitationalproddingthat
madetheorbitsofSaturnandJupiterlesscircular.Inturn,theorbitsofNeptune
andUranuswereperturbedandmovedoutwards,alsobecomingmoreelliptical.
NeptunecouldhaveevenbeguninsidetheorbitofUranusandthenovertakenit
inamigrationoutward,andbothplanets,particularlythefartherone,would
havescatteredsmallicybodies,sometowardstheinnerSolarSystem.
Meanwhile,thegravityandmovementofJupiterwouldflingsomeasteroidsinto
theinnerSolarSystemandpushothersaway.Duringthistime,theEarthmust
havesufferedevenmoreimpactsthantheMoonbecauseofEarth’sgreatersize
andgravity.Eventually,afterwreakinghavoc,theorbitsofthegiantplanets
wouldhavesettleddown.
Theoriginoflife
Quitehowlifearoseisunknown.ItmayhaveoriginatedonEarthoritwas
carriedherebyspacedustormeteorites.Thelatterideaiscalledpanspermia,
anddoesn’tsolvehowlifeoriginated,butpushestheproblemelsewhere.Also,
theremaybedifficultieswithsurvivaloverlongtransittimesiflifecamefrom
aroundotherstars.Forthesereasons,we’llconcentrateonaterrestrialoriginof
life.
Thereiswideagreementthattheoriginoflifewouldhavebeenprecededbya
periodofchemicalevolution,orprebioticchemistry,duringwhichmore
complexorganicmoleculeswereproducedfromsimplerones.Theideagoes
backtothe19thcentury.In1871,CharlesDarwinimaginedthatsuchchemistry
mighthaveoccurredina‘warmlittlepond’,accordingtoalettertothebotanist
JosephHooker:
It is often said that all the conditions for the first production of a living organism are now
present,whichcouldeverhavebeenpresent.—Butif(andohwhatabigif)wecouldconceive
in some warm little pond with all sorts of ammonia and phosphoric salts,—light, heat,
electricityetc.present,thataproteincompoundwaschemicallyformed,readytoundergostill
more complex changes, at the present day such matter would be instantly devoured, or
absorbed,whichwouldnothavebeenthecasebeforelivingcreatureswereformed.
Thus,Darwinconcludedthatlifeisunlikelytooriginatetodaybecause
organismsarecontinuallyeatingthechemicalcompoundsthatareneeded.On
theotherhand,beforelifeexisted,chemicalconditionsfortheoriginoflife
wouldhavebeenprevalent.
Inthe1920s,theRussianbiochemistAlexanderOparinandtheBritishbiologist
J.B.S.Haldanebothrecognizedthattheenvironmentunderwhichlifearose
wouldhavelackedoxygen.Earth’soxygen-richatmosphereisaproductof
photosynthesisbyplants,algae,andbacteria.Anoxygen-freeatmospherewould
havebeenbettersuitedtoprebioticchemistrybecauseoxygenconvertsorganic
matterintocarbondioxide,whichpreventsthebuild-upofcomplexmolecules.
Indeed,acontemporarygeologist,AlexanderMacGregor,reportedin1927that
hehadfoundsedimentaryrocksdatingfromwithintheArchaeanAeon(3.8–2.5
Ga)thatshowedthattheancientatmospherelackedoxygen.Inparticular,
MacGregorobservedthatironmineralsinwhatisnowZimbabwewere
unoxidized,unliketherust-colouredironoxidesinmodernsedimentsthatare
producedwhenatmosphericoxygenreactswithiron-containingminerals.Today,
MacGregorsdeductionissupportedbymanyotherdata(Chapter4).
OparinandHaldanealsoproposedthatgasesinEarth’searlyatmospherewould
havebeenconvertedbyultravioletsunlightorlightningintoorganicmolecules.
Inthe1950s,suchideasweretestedwhenStanleyMiller,astudentoftheNobel
Prize-winningchemistHaroldUrey,devisedanexperimentattheUniversityof
Chicago.Ablendofammonia,methane,hydrogen,andwatervapourwasputin
aflaskandelectricsparkspassedthroughthegastosimulatelightning.Miller
foundthatyellowishwatercondensedoutatthebottomoftheflask.Thisdark
residuecontainedorganicmolecules,includingvariousaminoacids—the
buildingblocksofproteins.Atthetime,theMiller–Ureyexperimentwas
trumpetedinthemediaasvirtuallysolvingtheproblemoftheoriginoflife.Life
wassaidtostartina‘primordialsoup’generatedbyatmosphericchemistry.In
1953,whenMillerwroteuphisresults,acommonview(datingbacktoOparin
andHaldane)wasthatgeneticmaterialwasprotein,butamonthbeforeMillers
papercameout,DNAwasidentifiedastherealbasisofheredity.Subsequently,
experimentsintheMiller–Ureygenrehavealsoproducedmoleculescontaining
hexagonalringsofcarbon,nitrogen,andhydrogenatoms,whicharethekindof
ringsfoundinDNA.
However,geochemistshaveraiseddoubtsabouttheMiller–Ureyexperiment
becausetheyarguethattheEarth’searlyatmospherewasprobablynotas
hydrogenrichasMillerassumed.VolcanoessupplytheEarth’satmospherewith
gasesoverlongtimescalesbutmostvolcanicgasissteam,i.e.watervapour
(H
2
O),withanaverageoflessthan1percenthydrogen(H
2
).Similarly,volcanic
carboncomesoutascarbondioxide(CO
2
)ratherthanthehydrogenatedformof
methane(CH
4
),whilenitrogenisemittedasdinitrogen(N
2
)insteadofammonia
(NH
3
).
ItisthoughtthattheatmosphereintheHadeanwasmostlymaintainedfrom
gasesreleasedthroughvolcanism,sotheairshouldhaveconsisted
predominantlyofCO
2
andN
2
.Ofcourse,itdependsonhowfarbackyougo.
WhentheEarthwasforming,thevaporizationoflargeimpactorswouldhave
producedhydrogen-richatmospheresifimpactorsweresufficientlyrichiniron
toprovidetherightkindofchemistrytostabilizehydrogenintheimpact
explosion.Earlyon,mostoftheatmospherewassteamandoceanshadnotyet
condensed.Afteroceansformed,atmospheresproducedbyimpactvaporization
wouldhavebeenephemeral.Becauseofuncertaintiesaboutthecompositionof
theHadeanatmosphere,astrobiologistshaveproposedsourcesoforganiccarbon
otherthanthatpresumedbyMillerandUrey.
Onealternativeisthatorganiccarboncamefromspace.Somecarbon-rich
meteoritescontainaminoacids,alcohols,andotherorganiccompoundsthat
perhapscouldhaveseededEarthwiththematerialsneededforprebiotic
chemistry.Forexample,theMurchisonmeteorite,whichfellonMurchison,
Australia,in1969,containsover14,000differentmolecules.Also,
micrometeoriteparticlesthatare0.02–0.4mminsizecurrentlybringabout30
millionkgoforganiccarbontotheEarthperyear.Organiccarboncanfallintact
totheEarth’ssurfacebecausetinyparticlesdon’tnecessarilyburnupinthe
atmosphere.Furthermore,theinteriorsofcometsarerichinorganicmaterialand
mayhaveplayedasimilarroletometeoritesingettinglifestartediftheorganic
materialsurvivedcometaryimpactsontheEarth.
Anotherpossiblesourceoforganiccarbonisdeep-seahydrothermalvents.A
hydrothermalventisaspotwherehotwateremergesfromtheseafloor.New
seaflooriscreatedatmid-oceanridges,wheretheEarth’stectonicplatesseparate
andmagmarisesupfromthemantle,fillingthevoid.Seawaterflowsdown
throughcracksneartheridges,isheatedbythehotmagma,andrises,pickingup
substancesproducedbychemicalreactionsbetweenthewaterandrocks,suchas
hydrogen,hydrogensulphide,anddissolvediron.Whenthehotacidicwaterhits
cold(2°C)oceanwater,itproducesaplumeofprecipitatedparticlescontaining
dark-colouredpyrite,anironsulphidemineral(FeS
2
,whereFeisironandSis
sulphur).Thatiswhythesehydrothermalventsarecalled‘blacksmokers’.One
hypothesissuggeststhatlifeoriginatedonthesurfaceofsuchpyriteminerals.
However,blacksmokersareveryhot,around350°C,somostresearcherswho
favouradeep-seaoriginoflifehavefocusedonventsthatarecooler,around
90°C,alkalineratherthanacidic,andlocatedfurtherfromthemid-oceanridges.
Beneathalkalinevents,hydrogenreleasedinreactionsbetweenwaterandrock
cancombinewithcarbondioxidetomakemethaneandbiggerorganic
molecules.IntheHadean,tinyporesinthemineralstructuresthatgrewupfrom
ventscould,inprinciple,havecontainedandconcentratedsimplemoleculesto
allowthemtoreactandmakemorecomplexprebioticmolecules.
Also,inalkalinevents,becausethefluidemergingismorealkalinethan
seawater,thereisanaturalgradientinpHthatissimilartothatincells.Iflife
originatedinsuchanenvironment,itmightexplainwhyenergyproductionis
intimatelytiedtopHgradients.
Lifegeneratesenergyfrommicroscopicelectricalmotorsthatareembeddedin
cellmembranesandrunoffelectricalcurrentsdrivenbypHgradientsacrossthe
membranes.Itisimpossibleforwordstodojusticetotheseamazingmolecular
machines,soIsuggestthatthereadersearchtheInternetfor‘ATPsynthase
animations’.Essentially,metabolicenergyisusedtocreateadifferentpHon
eachsideofacellmembranewiththeoutsideusuallymoreacidicthanthe
inside.Thisdifferentialisaprotongradient,becausepHisaninversemeasure
oftheconcentrationofpositivelychargedhydrogenions(protons)insolution—
thegreatertheconcentration,thelower(andmoreacidic)thepH.Theproton
gradientisessentiallyabattery.Whendischarged,electricalcurrentflows
throughthemolecularturbineinthecellmembraneandgeneratesmoleculesthat
storeenergy.Thesemoleculesofadenosinetriphosphateor‘ATP’aretheenergy
carriersinallcells.Forexample,ahumantypicallycontains250gofATPand
usesupabodyweight’sworthofATPeverydayasATPismadeused,and
remade.
TheaboveprocessofgeneratingATPiscalledchemiosmosis,whichcontainsthe
word‘osmosis’,meaningwatermovingacrossamembrane,becausetheproton
movementisanalogous.Chemiosmosisissoweirdcomparedwiththefamiliar
ideaofgeneratingenergyfromchemicalreactionsthatwhenitwasproposedin
1961byBritishbiochemistPeterMitchellmostbiochemistswerehostile.
Eventually,MitchellwasvindicatedwithaNobelPrizein1978.Giventhat
chemiosmosisisuniversaltoterrestriallifeandpresumablyrelatedtoitsorigin,
itisinterestingtowonderwhetheriongradientsmightbefundamentalforlife
anywhere.
Currently,thesourceoftheorganiccarbonthatledtotheoriginoflife—the
atmosphere,space,orhydrothermalvents—isunresolved.Givensuchasource,
however,organiccompoundsmusthavebeenconcentratedforchemical
reactionstohappen,whichcouldhaveoccurredwhenfilmsformedonmineral
surfaces,perhapsarisingfromdryingorcoldness.Thenthesimpleorganics
couldjoinuptomakemorecomplexmolecules.Butchemicalsystemsneed
severalotherfeaturesbeforetheycanbeconsidered‘alive’.Themainonesarea
metabolism,enclosurebyasemi-permeablemembrane,andreproductionthat
incorporateshereditywithagenome.
Withregardtothelastcharacteristic,amoretractableissueforlaboratory
experimentationthantheoriginoflifeitselfisahypothesizedstageofveryearly
life,calledtheRNAWorld,inwhichRNAprecededDNAasthegeneticmaterial.
Inmoderncells,DNA(deoxyribonucleicacid)providesaninstructionsetthatis
transcribedintoRNA(ribonucleicacid)moleculesthatcarrytheinformation
neededtomakespecificproteins.DNAissimilartoRNAbutmorecomplicated,
soitislogicalthatRNAevolvedfirst.Moreover,inthe1980sitwasdiscovered
thatsomeRNAmolecules,knownasribozymes,canactascatalysts.Soitis
plausiblethatRNAoncecatalyzeditsself-replicationandassemblyfromsmaller
molecules.Atthenextstage,RNAwouldbegintomakeproteins,someofwhich
wouldbebettercatalyststhanRNAitself.Eventually,DNAwouldreplaceRNA
becauseitismorestableandcanbelarger,bothofwhichconferreproductive
advantages.
Anothercriticalfeatureintheoriginoflifewastheencapsulationofagenomein
acellmembrane.Thiswasbeneficialforacoupleofreasons:first,toincrease
ratesofreactionbyconcentratingbiochemicals;andsecond,togivean
evolutionaryadvantagetoself-replicatingmolecules.Forexample,anRNA
genomethatproducedausefulproteincouldkeepittoitselfinsideacell,
therebyfavouringitsprogeny.
So-called‘pre-cell’structureshavebeenmadeinthelab.Sphericalstructures
formspontaneouslywhenoilsaremixedwithwater,whichissomethingwe’re
allfamiliarwithinthekitchen.Suchoilyorfattymembranesplausiblyledtothe
membranesthatsurroundmoderncellsandweresurfacesforprebiotic
chemistry.
Subsequenteventsmayhaveoccurredasfollows.RNAtookupresidenceinpre-
cells,cellswithanRNAgenomeevolved,andthenmoderncellswithaDNA
genometookoverfromRNA.Atsomepoint,metabolismalsoevolved,perhaps
afterRNAWorld.However,thereremainsanongoingdebateaboutwhichcame
first,metabolismorgeneticreplication.Also,theactuallocation,conditions,and
evolutionarystepsfortheoriginofliferemainuncertainandareanareaof
researchripeforsignificantprogress.
Chirality(ortheartofclappingwithonehand)
Adescriptionoflife’soriginwouldbeincompletewithoutmentioningarelated
aspectofbiochemistry:forbiomoleculesthatcomewithtwomirror-image
structures,lifeonEarthusesonlyoneofthestructures,notboth.Thepropertyof
havingmirror-imagesymmetryiscalledchirality,whichcomesfromtheGreek
cheirfor‘hand’,giventhefactthataperson’shandsaremirrorimagesand
cannotfitontopofoneanother(thesamewayup).Ifyou’renotconvincedthat
aleft-handstructureistrulydifferentfromarightone,trywearingyourshoeson
thewrongfeetallday!
Chiralityarisesinbiomoleculesifacentralcarbonatomissurroundedbyfour
differentgroupsofatoms.Thegroupsnaturallyarrangethemselvesina
tetrahedron,whichisapyramidwithfourtriangularfaces.Ifwenumberthe
groupswitha‘1’atthetopofthetetrahedron,thethreetetrahedralfeetcanbe
numberedclockwiseorcounterclockwiseasshown:
Thetwomoleculesinthepictureare‘handed’becauseonecannotbe
superimposedontopoftheother.Suchmoleculesarecalledenantiomersfrom
theGreekenantiosmorphe,foroppositeshape.Let’sconsideraspecific
enantiomer,theaminoacidalanine.Itwouldhave‘1’asahydrogenatom,‘2’as
acarboxylgroup(COOH),‘3’asanaminegroup(NH
2
),and‘4’asamethyl
group(CH
3
).Byconvention,withtheclockwisenumberingofgroups2to4,
alanineissaidtobeintheright-handedorD-formaftertheGreekdextrofor
right.Withthecounterclockwisearrangement,alanineisintheLformfromthe
Greeklaevoforleft.Remarkably,lifeonEarthusesL-aminoacidsinproteins,
almostentirely,andD-sugarsalmostexclusivelyaswellwhetherinDNA,cell
walls,orsynthesis.(Ahandymnemonicis‘lads’forleftamino,dextrosugars).
However,whenachiralsubstanceismadeinthelaboratory,equalproportionsof
left-andright-handedformsareusuallyproduced,calledaracemicmixture.
Chiralmoleculescanhaveverydifferenteffectsdespitetheiridenticalchemical
formulae.Anexampleistheinfamousdrugthalidomide.Theright-handedform
curesmorningsicknessinpregnantwomen,buttheleft-handedversioninduces
severebirthdefects.Amorepleasantexampleistheflavouringlimonene:theD-
formtasteslikelemonsbuttheL-formisorange-flavoured.
Whatcausedthecommonchirality,orhomochirality,sharedbyalllife?There
aretwogeneralhypotheses.Oneisthattheorganicmoleculesfromwhichlife
startedhadaslightexcessofoneenantiomerthatwassubsequentlyamplified.
Physicalprocessesinvokedtoinducethisexcessincludepolarizedlightfrom
starsthatmightcauseorganicmoleculesthatseededtheEarthtodevelopachiral
biasduringtheirsynthesisinspace;alternatively,polarizedradiationfrom
radioactivedecaymightgenerateanenantiomericexcess.Aproblemwithsuch
radiation-relatedideasisthattheexcessisoftensmall,lessthan1percent.The
secondhypothesisisthatprebioticchemistrypreferentiallyassembledonlyone
oftheenantiomers.Forexample,adsorptiononamineralsurfacemighthave
chiralselectivity.Chemicalkineticsisgenerallymoreefficientforapure
substancethanamixture,sothismightcausefurtheramplification.One
biologicalprocess—reproductionofagenome—mayrequirehomochirality.
HomochiralitywasprobablyneededforthereplicationintheRNAWorldmodel
becauseonlyreplicatedRNAstrandsofthesamechiralitywouldlineupwithan
RNAtemplatestrand.
Signsoftheearliestlife
Althoughtheoriginofliferemainsobscure,intheoldestsedimentaryrockson
Earthwefindpossiblesignsoflife.Perhapsit’snocoincidence,buttheserocks
(mixedwithvolcanicflows)datefromtheendoftheLateHeavyBombardment,
around3.8Ga.TheyarefoundinIsua,intheinteriorofsouth-westGreenland.
Theyformadark,forebodinglandscapethatlookssomethinglikeMordor,albeit
icy,andtheyareremarkablefortellingusmuchabouttheearlyEarth.Thereis
gravelthatwasroundedbytheactionofwater.Also,somesedimentsthatwere
laiddownundertheseacontainflakesofcarbonintheformofgraphite.Isotopic
analysissuggeststhatthiscarbonwasoncepartofmicrobeslivingintheoceans.
Carbonhastwostableisotopes,carbon-12andcarbon-13.Lifetendsto
concentrateafewpercentmorecarbon-12atomsinitstissuesthancarbon-13
becausemoleculescontaininglightercarbon-12atomsreactfasterin
biochemistry.ThegraphiteinIsuaisenrichedincarbon-12byabout2percent,
whichissimilartotheproportioninmarinemicrobes.Sotheinferenceisthat
marinemicrobesdiedandsankintothesedimentswherethecarbonwas
subsequentlycompressedandconvertedintographite.
Betterpreservedevidenceforearlylifeisfoundinnorth-westAustraliaand
datesfromabout3.5Ga.Inthe1980s,mycolleagueRogerBuickfoundfossil
stromatolitesofthisagenearNorthPoleinnorth-westernAustralia(Fig.2).An
Aussiejokernamedtheplace‘NorthPole’becauseitisoneofthehottest,most
sun-bakedlocationsonEarth.Stromatolitesarelaminatedsedimentarystructures
madebyphotosynthesizingmicrobialcommunitiesinwaterthatisshallow
enoughtoreceivesunlight.Oftenstromatolitestaketheformofdomesof
wrinklylaminations.Thesedomesarebuiltupfromtrappedsedimentassheets
ofmicrobes,calledmicrobialmats,growingupwardstowardssunlight.On
modernstromatolites,thelivingpart—themat—isjustaveneerabouta
centimetrethickwithatextureliketofuthatsitsatopaccumulatedmineral
layers.IfyoucouldtravelbacktotheArchaeanEarth,youwouldseecoastlines
coveredinstromatolitesallovertheworld.Today,stromatolitesarerarebecause
certainfishandsnails(whichdidn’texistintheArchaean)feedonthemicrobial
matsthatmakethem.Consequently,modernstromatolitesarefoundonlyin
lagoonsorlakeswherethewateristoosaltyforsuchvoraciousanimals.
2.Left:Cross-sectionoftheworld’soldestfossilstromatolitesintheDresser
Formation,NorthPole,Australia.Right:Aplanviewofthebeddingplane
ofthestromatolites,showingtheirtops.Thelenscapis5cmdiameterfor
scale
SomescepticswonderiftheNorthPolestructurescouldhaveformedwithout
lifeandsoarenotreallystromatolites,butmostastrobiologistsaccepttheir
organicheritagebecausetheyhavevariousfeaturesweexpectofbiology.The
fossilstromatoliteshavelaminationswithextremelyirregularwrinklesthatare
difficulttoexplainasfoldscreatedbypurelyphysicalprocesses.Also,the
laminationsthickenatthetopofconvexflexures,whichisjustwhathappens
whenphotosyntheticmicrobesgrowwherethereismoresunlight.Introughs
betweenflexurestherearefragmentsthatareplausiblyinterpretedaspiecesof
microbialmatthatwererippedupbywavesinthesea.Thefragmentscontain
thin,wrinklylayersthatincorporateorganiccarbon.Thecombinationofsuch
observationssuggeststhattheNorthPolestromatolitesareindeedtheoldest
fossilstructuresvisibletothenakedeye.
Tofindevidenceforpastlife,wecanalsolookforindividualdeadbodiesof
single-celledorganisms,sinceonlymicrobesexistedsofarbackinEarthhistory.
Suchmicrofossils,whichcanonlybeseenwithamicroscope,arefoundin
sedimentaryrockssuchaschert(afine-grainedsilica-richrock;flintisafamiliar
example)andshales,whicharefine-grainedsedimentaryrocksthatwereonce
muds.
Geologistswhostudymicrofossilstakerocksamplesfromlikelysettingsbackto
thelabwheretheyslicethemupandhopethatsomethingshowsupunderthe
microscope.It’sahit-and-missaffair.Theproblemofdistinguishinglifefrom
non-lifealsorearsitsuglyheadagainbecausemineralgrainssometimeslook
likefossilcells.Consequently,microfossilsaremoreconvincingiftheyshow
signsofcelldivision,colonialbehaviour,orfilamentousstructuresthatoccur
whencellsjoinupinaline.Microfossilscanalsobetestedtoseeiftheycontain
organiccarbon.
TheoldestincontestablemicrofossilsoccurinSouthAfricafrom2.55Ga.They
areconvincingbecausetheyarefoundinfossilizedmatsoffilamentsthat
containorganiccarbonandincludesomemicrobialformsimmobilizedintheact
ofcelldivision.ThereareolderpossiblemicrofossilsinSouthAfricadatedat
3.2–3.5Gathatarespheroidal,containorganiccarbon,andappeartoshowcell
division,buthavenootherbiologicalattributes.Similarmicrofossilcoloniesin
3.4GasandstoneoftheStrelleyPoolFormationinnorth-westAustraliaalso
possessorganiccarbonwithisotopescharacteristicoflife.
Theworld’smostcontroversialmicrofossilscomefromtheApexChertrock
formationinnorth-westAustralia,withanageof3.5Ga.Thesewereidentified
asthe‘world’soldestmicrofossils’inthe1990sbyBillSchopf,oftheUniversity
ofCalifornia,LosAngeles.Thestructuresappearasblacktodarkbrownstreaks
thatappeartobepartitionedintocell-likesections.SchopfgavethemLatin
namesimplyingthattheywereextinctcyanobacteria,whicharephotosynthetic
bacteria.In2002,arancorousdisputearosewhenMartinBrasierofOxford
Universityre-examinedthesamplesandshowedthattheydidn’tlooklike
microbialfilamentsinthreedimensions.Brasieralsonotedthatthechertwasnot
sedimentarybutatypeproducedinahydrothermalvent,whichisanunlikely
placetofindcyanobacteriathatneedsunlight.Sincethen,furtherinvestigation
hassuggestedthatthefilamentsarereallyfracturesthatarepartiallyfilledwith
haematite,anironoxide,whichprovidesthedarkcolour.However,organic
carbonisdispersedinthechertoutsidethefilamentsandthecontroversy
continues.
Sofar,we’vediscussedisotopes,microfossils,andstromatolites.Further
possibleindicatorsofearlylifearebiomarkers,whicharerecognizable
derivativesofbiologicalmolecules.We’reallfamiliarwiththeconceptthat
fossilskeletonsallowustodistinguishaTyrannosaurusrexorTriceratops.On
themicroscopicscale,microbescanleavebehindremnantsofindividualorganic
molecules,consistingofskeletalringsorchainsofcarbonatoms.These
‘molecularbackbones’comefromparticularmoleculesthatarefoundonlyin
certaintypesofmicrobe.Sobiomarkersnotonlyconfirmthepresenceofpast
lifebutcanalsoindicatespecificformsoflife.
Unfortunately,likesomemicrofossils,ancientbiomarkersarealsomiredin
controversy.Theoldestreportedbiomarkersdatefromabout2.8Gaandappear
toshowmoleculessuggestingthepresenceofoxygen-producingcyanobacteria.
Butthesampledrocksmayhavebeencontaminatedbyyoungerorganic
material.Morecertainbiomarkersarefoundinrocksfrom2.5Gainsidetiny
fluidinclusionsthatappearuncontaminated.
Despitethelimitationsofsomeevidence,theoverallrecordshowsthatEarth
wasinhabitedby3.5Ga—within1billionyearsofitsformationandshortlyafter
heavyimpactbombardment,whichsuggeststhatlifemightoriginatefairly
quicklyongeologicaltimescalesonsuitableplanets.Thismeansthatit’snotout
ofthequestionthatMars,whichwethinkhadageologicallyshortwindowof
habitability(Chapter6),mayalsohaveevolvedlife.
Chapter4
Fromslimetothesublime
HowdidEarthmaintainanenvironmentfitforlife?
Howeverlifestarted,onceestablished,itpersistedforover3.5billionyearsand
evolvedfrommicrobialslimetothesophisticationofhumancivilization.During
thisperiod,theEarthmaintainedoceansand,forthemostpart,amoderate
climate,eventhoughtherewasanincreaseofabout25percentintheamountof
sunlight.ThegradualbrighteningoftheSunisaconsequenceofthewaythatthe
Sunshinesonthemainsequence.Whenfourhydrogennucleiarefusedintoone
heliumnucleusintheSun’score,therearefewerparticles,somaterialabovethe
corepressesinwardstofillavailablespace.Thecompressedcorewarmsup,
causingfusionreactionstoproceedfaster,sothattheSunbrightensabout7–9
percenteverybillionyears.ThistheoryisconfirmedbyobservationsofSun-
likestarsofdifferentages.
IftheEarthhadpossessedtoday’satmosphereatabout2Gaorearlier,thewhole
planetshouldhavebeenfrozenunderthefainterSun,butgeologicalevidence
suggestsotherwise.ThispuzzleisthefaintyoungSunparadox.Thereis
evidenceforliquidwaterbacktoatleast3.8Ga,whichincludes,forexample,
thepresenceofsedimentaryrocksthatwereformedwhenwaterwashedmaterial
fromthecontinentsintotheoceans.
TherearethreewaystoresolvethefaintyoungSunparadox.Themostlikely
explanationinvolvesagreatergreenhouseeffectinthepast.Anothersuggestion
isthattheancientEarthasawholewasdarkerthantodayandabsorbedmore
sunlight,althoughthere’sscantsupportingevidence.Athirdideaisthatthe
youngSunshedlotsofmaterialinarapidoutflow(solarwind)tospacesothat
theSun’scorewasn’tascompressedandheatedovertimebyoverlyingweight
asassumedabove.Ifthemasslosswasjustright,theSuncouldhavestartedout
asbrightastoday.However,observationsofyoungSun-likestarspresentlydon’t
supportthethirdhypothesis.
Inconsideringthefirstidea,weneedtoappreciatethattheatmospherewarms
theEarthtoanextentdependingonatmosphericcomposition.Withoutan
atmosphere(andassumingthattheamountofsunlightthattheEarthreflects
stayedthesame),theEarth’ssurfacewouldbeachilly–18°C.Instead,today’s
averageglobalsurfacetemperatureis+15°C.The33°Cdifference(=18+15)is
thesizeofEarth’sgreenhouseeffect,whichisthewarmingcausedbythe
atmosphere.
Howdoesthegreenhouseeffectwork?Aplanet’ssurfaceisheatedbyvisible
sunlight,causingittoglowintheinfraredjustasyourwarmbodyshinesatthose
wavelengths.Anatmospheretendstobemostlyopaquetotheinfraredradiation
comingupfromthesurfacebelow,soitabsorbstheinfraredenergyandwarms.
Becausetheatmosphereiswarmitalsoradiatesintheinfrared.Someofthis
radiationfromtheatmospheretravelsbackdowntotheplanet.Sothesurfaceof
aplanetiswarmerthanitwouldbeintheabsenceofanatmospherebecauseit
receivesenergyfromaheatedatmosphereinadditiontovisiblesunlight.
At3.5Ga,whentheSunwas25percentfainter,a50°Cgreenhouseeffectwould
havebeenneededtomaintainthesameglobalaveragetemperatureonEarththat
weenjoytodaywitha33°Cgreenhouseeffect.Astrongergreenhouseeffectis
possibleiftherehadbeengreaterlevelsofgreenhousegases,whicharethose
responsibleforabsorbinginfraredradiationcomingfromtheEarth’ssurface.
Today,watervapouraccountsforabouttwo-thirdsofthe33°Cgreenhouse
effect,andcarbondioxide(CO
2
)accountsformostoftherest.However,water
vapourcondensesasrainorsnow,soitsconcentrationisbasicallyaresponseto
thebackgroundtemperaturesetbyatmosphericCO
2
,whichdoesn’tcondense.In
thisway,CO
2
controlstoday’sgreenhouseeffecteventhoughitslevelissmall.
Around1700,therewereabout280partspermillionofCO
2
inEarth’s
atmosphere(meaningthatinamillionmoleculesofair,280wereCO
2
),whilein
2010therewere390partspermillion.Sinceindustrialization,CO
2
hasbeen
releasedfromdeforestationandburningfossilfuelssuchasoilorcoal.Inthe
lateArchaean,anupperlimitontheCO
2
amountoftenstoahundredtimes
higherthanthepre-industriallevelisdeducedfromthechemicalanalysesof
palaeosols,whicharefossilizedsoils.ButevensuchlevelsofCO
2
werenot
enoughtocounterthefaintyoungSun.
Infact,theArchaeanatmospherehadlessthanonepartpermillionofmolecular
oxygen(O
2
),whichimpliesthatmethane(CH
4
)wasanimportantgreenhouse
gas.Today,atmosphericmethaneisatalowlevelof1.8partspermillion
becauseitreactswithoxygen,whichisthesecondmostabundantgasintheair
at21percent.(Mostoftoday’sairisnitrogen(N
2
),whichis78percent,but
neitheroxygennornitrogenaregreenhousegases.)IntheArchaean,thelackof
oxygenwouldhaveallowedatmosphericmethanetoreachalevelofthousands
ofpartspermillion.Methanewouldn’taccumulatewithoutlimitbecauseitcan
bedecomposedbyultravioletlightintheupperatmosphere.Subsequent
chemistryinvolvingmethane’sdecompositionfragmentscangenerateother
hydrocarbons,i.e.chemicalsmadeofhydrogenandcarbon,includingethanegas
(C
2
H
6
)andasmogofsootyparticles.Acombinationofmethane,ethane,and
carbondioxide—plusthewatervapourthatbuildsupinresponsetothe
temperaturesetbythesenon-condensablegreenhousegases—wouldhave
providedenoughgreenhouseeffecttooffsetthefainterSun.Thisassumesthat
therewasasourceofmethanefrommicrobiallife,justliketoday,whichis
plausiblebecausethemetabolismformethanegenerationisancient(seeChapter
5).
Ofcourse,wecouldaskwhattheEarth’searlyclimatewaslikebeforelife.In
thatcase,CO
2
probablycontrolledthegreenhouseeffect,astoday.Infact,
throughoutmuchofEarthhistory,therehasbeenageologicalcycleofCO
2
that
regulatedclimateontimescalesofaboutamillionyears.(Itstilloperatestoday
butisfartooslowtocounteracthuman-inducedglobalwarming.)Essentially,
atmosphericCO
2
dissolvesinrainwaterandreactswithsilicaterocksonthe
continents.Thedissolvedcarbonfromthischemicalweatheringthentravels
downriverstotheoceans,whereitendsupinrocksontheseafloor,suchasin
limestone,whichiscalciumcarbonate(CaCO
3
).Ifdepositionofcarbonateswere
allthatwerehappening,theEarthwouldloseatmosphericCO
2
andfreeze,but
thereisamechanismthatreturnsCO
2
totheatmosphere.Seafloorcarbonatesare
transportedonslowlymovingoceanicplatesthatdescendbeneathotherplatesin
theprocessofsubduction.AnexampletodayistheSouthPacific‘Nazca’plate,
whichisslidingeastwardsunderChile.Carbonatesaresqueezedandheated
duringsubduction,causingthemtodecomposeintoCO
2
.Volcanism(where
rocksmelt)andmetamorphism(whererocksareheatedandpressurizedbut
don’tmelt)releaseCO
2
.ThewholecycleofCO
2
lossandreplenishmentis
calledthecarbonate–silicatecycleandbehaveslikeathermostat.Iftheclimate
getswarm,morerainfallandfasterweatheringconsumeCO
2
andcooltheEarth.
IftheEarthbecomescold,CO
2
removalfromthedryairisslow,soCO
2
accumulatesfromgeologicalemissions,increasingthegreenhouseeffect.
Thecarbonate–silicatecycleprobablyregulatedclimatebeforelifeoriginated.It
thenlikelyplayedanincreasinglyimportantroleasathermostatafterlevelsof
methanegreenhousegasdeclinedintwostepswhenatmosphericoxygen
concentrationsincreased,firstaround2.4Gaandthen750–580Ma(Ma=
millionsofyearsago).
AcaveatisthatthecyclemayhaveoperateddifferentlyintheHadeanand
possiblytheArchaeanbecauseplatetectonics—thelarge-scalemotionof
geologicplatesthatrideonconvectioncellswithinthemantlebelow—probably
hadadifferentstyle.Radioactiveelementsinthemantlegenerateheatwhenthey
decay,andmoreweredecayingontheearlyEarth.So,ontheonehand,ahotter,
lessrigidHadeanmantleshouldhaveallowedoceaniccrusttosinkmore
quickly.Ontheotherhand,ahottermantleshouldhaveproducedmoremelting
andthicker,warmeroceaniccrustthatwaslesspronetosubduct.Overall,the
presenceofgranitesinferredfromzircons(Chapter3)impliesthatcrustmust
havebeenburiedsomehowbecausegraniteisproducedwhensunkencrust
melts.However,exactlyhowtectonicsoperatedontheearlyEarthremainsan
openquestion.
TheGreatOxidationEvent:asteptowardscomplex
life
ThemostdrasticchangesoftheEarth’satmosphericcompositionhavebeen
increasesinoxygen,whichwerejustasimportantfortheevolutionoflifeas
variationsingreenhousegases.FormostofEarth’shistory,oxygenlevelswere
solowthatoxygen-breathinganimalswereimpossible.TheGreatOxidation
Eventiswhentheatmospherefirstbecameoxygenated,2.4–2.3Ga.However,
oxygenlevelsonlyreachedsomewherebetween0.2to2percentbyvolume,not
today’s21percent.Largeanimalswereprecludeduntilaround580Maafter
oxygenhadincreasedasecondtimetolevelsexceeding3percent(Fig.3).
Nearlyallatmosphericoxygen(O
2
)isbiological.Atinyamountisproduced
withoutlifewhenultravioletsunlightbreaksupwatervapourmolecules(H
2
O)
intheupperatmosphere,causingthemtoreleasehydrogen.Netoxygenisleft
behindwhenthehydrogenescapesintospace,therebypreventingwaterfrom
beingreconstituted.Butabioticoxygenproductionissmallbecausetheupper
atmosphereisdry.Instead,themajorsourceofoxygenisoxygenic
photosynthesis,inwhichgreenplants,algae,andcyanobacteriausesunlightto
splitwaterintohydrogenandoxygen.Theseorganismscombinethehydrogen
withcarbondioxidetomakeorganicmatter,andtheyreleaseO
2
aswaste.
3.Theapproximatehistoryofatmosphericoxygen,basedongeologic
evidence(ppm=partspermillion;ppt=partspertrillion)
Another,moreprimitive,typeofmicrobialphotosynthesisthatdoesn’tsplit
waterorreleaseoxygenisanoxygenicphotosynthesis.Inthiscase,biomassis
madeusingsunlightandhydrogen,hydrogensulphide,ordissolvedironin
hydrothermalareasaroundvolcanoes.Today,microbialscumgrowsthiswayin
hotsprings.
Beforeplantsandalgaeevolved,theearliestoxygen-producingorganismswere
similartomoderncyanobacteria.Cyanobacteriaarebluish-greenbacteriathat
teemintoday’soceansandlakes.DNAstudiesshowthatacyanobacteria-like
microbewastheancestorofplants,algae,andmoderncyanobacteria.
Consequently,wemightsupposethattheatmospherebecameoxygenatedonce
cyanobacteriaevolved.However,evidencesuggeststhatcyanobacteriawere
producingoxygenlongbeforeitfloodedtheatmosphere.Aplausibleexplanation
isthatreductants,whicharechemicalsthatconsumeoxygen,atfirstrapidly
overwhelmedtheoxygen.Reductantsincludegasessuchashydrogen,carbon
monoxide,andmethanethatcomefromvolcanoes,geothermalareas,and
seafloorvents.Distinctiveiron-richsedimentaryrockscalledbandediron
formationsdatingfromtheArchaeanshowthattherewasconsiderabledissolved
ironintheArchaeanocean,unliketoday’socean,whichwouldhavealsoreacted
withoxygen.
Thefirstsignsofphotosytheticoxygenappearabout2.7–2.8Gaaccordingto
evidencefromstromatolitesandthepresenceofchemicalsthatbecamesoluble
byreactingwithoxygen.Innorth-westAustralia,stromatolitesinrockscalled
theTumbianaFormationonceringedancientlakes.Intheory,microbesmight
havebuiltsuchstromatolitesusinganoxygenicphotosynthesis,butthere’sno
evidenceforhydrothermalemissionsneededforthismetabolism.Instead,
cyanobacteriausingoxygenicphotosynthesisprobablyconstructedthe
stromatolites,consistentwithtuftsandpinnaclesinthestromatolitesthatare
producedwhenfilamentsofcyanobacteriaglidetowardssunlight.Furthermore,
molybdenumandsulphur,whichareelementsonlysolublewhenoxidized,
becomeconcentratedinseafloorsedimentsafter2.8Gatolevelsthatare
possibleifmicrobesoxidizedtheseelementsonlandusinglocalsourcesof
oxygensuchasstromatolites.
Anotherreasonwhyoxygendidn’timmediatelyaccumulateintheEarth’s
atmosphereisthatitsproductionismostlyazero-sumprocess.Whena
moleculeofoxygenismade,anaccompanyingmoleculeoforganiccarbonis
generated,assummarizedinthefollowingequation:
carborndioxide+water+sunlight=organicmatter+oxygen
CO
2
+H
2
O+sunlight=CH
2
O+O
2
Theprocesseasilyreverses,i.e.organicmatterreactswithoxygentoregenerate
carbondioxideandwater.Also,theArchaeanatmosphere’slackofoxygen
meantthatmicrobescouldreadilyconvertorganiccarbonintomethanegas,as
happenstodayinsmelly,oxygen-freelakeorseafloorsediments.Intheair,
methanecouldreactwiththeoxygentorecreatewatervapourandcarbon
dioxide.Inbothcases—directreversalorindirectcancellationwithmethane—
nonetoxygenisproduced,despitethepresenceofphotosynthesis.
However,atinyfraction(about0.2percent,today)oforganiccarbonisburied
insedimentsandseparatedfromoxygen,whichpreventsthetwofrom
recombining.EveryorganiccarbonthatgetsburiedisequivalenttoonenetO
2
moleculethat’sfreedup.Ofcourse,this‘freed’oxygencaneasilyreactwith
manyothersubstancesbesidesorganiccarbon,includinggeologicalgasesand
dissolvedmineralssuchasiron.Nonetheless,atippingpointwasreachedwhen
theflowofreductantstotheatmospheredroppedbelowthenetflowofoxygen
associatedwithorganiccarbonburial,causingtheGreatOxidationEvent.
Afterwards,aplateauof0.2–2percentoxygenwasreachedprobablybecause
oxygenthatdissolvedinrainwaterreactedsignificantlywithcontinental
minerals,preventingitsfurtheraccrual.
Indeed,asurgeinoxidationistheevidencefortheGreatOxidation.‘Red
beds’—rust-colouredcontinentalsurfacesthatarisewhenironmineralsreact
withoxygen—appear.Today,reddishlandsurfacesarecommon,suchasinthe
Americansouth-west.Achangeinsulphurprocessingintheatmospherealso
occurred.BeforetheGreatOxidation,whentherewaslessthanonepartper
millionofatmosphericoxygen,red-andyellow-colouredparticlesofelemental
sulphurliterallyfelloutofthesky.Theyformedinchemicalreactionsatmany
kilometresaltitudewhensulphurdioxidegasfromvolcanoeswasbrokenupby
ultravioletlight.Thefallingsulphurparticlescarriedasulphurisotope
compositionintorocksthatindicatesatmosphericformation.AftertheGreat
Oxidation,oxygencombinedwithatmosphericsulphur,preventingelemental
particlesfromforming.Theskyclearedandtheisotopicsignaturevanishesfrom
sedimentaryrocks.
WiththeGreatOxidation,Earth’sstratosphericozonelayerformed.Thisregion
ofconcentratedozonebetween20and30kilometresinheightshieldstheEarth’s
surfacefromharmfulultravioletrays.Iftherewerenoozonelayer,youwouldbe
severelysunburnedintensofseconds.Ozoneisamoleculeofthreeoxygen
atoms(O
3
),whichcomesfromoxygen.Anoxygenmolecule(O
2
)issplitby
sunlightintooxygenatoms(O),whichthencombinewithotherO
2
moleculesto
makeozone(O
3
).
ThecauseoftheGreatOxidationisdebatedbutmustboildowntoeithera
globalincreaseinoxygenproductionfrommoreorganicburialoradecreasein
oxygenconsumption.Theproblemwiththefirstideaisthatorganicmatter
extractsthelightcarbonisotope,carbon-12,fromseawater,butseawatercarbon,
recordedinancientlimestones,doesn’tsteadilydecreaseincarbon-12content.
Thatleavesthesecondidea.
Butwhatwouldhavecausedtheflowofreductantstodiminish?Onepossibility
isthattherelativelylargeabundancesofhydrogen-bearinggasessuchas
methaneandhydrogen(whichareinevitableinanatmospherewithoutoxygen)
meantthathydrogenescapedrapidlyfromtheupperatmosphereintospace.The
lossofhydrogenoxidizedthesolidplanet.Oxidizedrockshavefewerreductants
sothisgraduallythrottledfurtherreleaseofreductants.Oxidationhappens
becauseyoucangenerallytraceescapinghydrogenbacktoitssourceinwater
(H
2
O),evenifhydrogenatomsgothroughmethane(CH
4
)orhydrogen(H
2
)
intermediaries.Sowhenhydrogenleaves,oxygengetsleftbehindwherethe
hydrogenoriginated.Watervapouritselfhastroublereachingtheupper
atmospherebecauseitcondensesintoclouds,butotherhydrogen-bearinggases
suchasmethanedon’tcondenseandsneakthehydrogenoutintospace.
AroughanalogyfortheGreatOxidationisanacid-basetitrationofthesort
performedinhighschoolbydrippingacidintoanalkalinesolution.Suddenly,
thesolutionchangescolour,typicallyfromcleartored.Theancientatmosphere
reachedasimilartransitionfromhydrogenrichtooxygenrich.Ratherthanacid
base,wecallsuchatitrationa‘redoxtitration’becauseit’sacompetition
betweenreductantssuchashydrogenandoxidantssuchasoxygen.
Aboringbillionyearsendedbytheadventofanimal
life
TheoceanandlandadjustedtothechangeoftheGreatOxidation,andbyabout
1.8Ga,atmosphericoxygenhadsettleddownandremainedbetween0.2and2
percentforanamazinglylongtime.Theevolutiontowardscomplexlifewas
slowperhapsbecauseanoxicwatersthatwereoftenrichindissolvedironor
hydrogensulphideunderlayamoderatelyoxygenatedsurfaceocean.Such
anoxicconditionsaretoxicforcomplexlife.Infact,evolutionwassosluggish
thattheintervalfrom1.8to0.8Gaiscalledtheboringbillion.Accordingtoone
scientificpaper,‘neverinthecourseofEarth’shistorydidsolittlehappentoso
muchforsolong’!
Actually,severalnotableeventsdidoccurintheotherwiseboringbillionandjust
beforeit.Around1.9Ga,thefirstfossilsofsingleorganismsappearthatare
visibletothenakedeye.Theseimpressionsofspiralcoilsofafewcentimetres
diameter(namedGrypania)wereprobablyseaweeds.InChina,acritarchs,a
typeoffossillargerthan0.05mminsizewithorganic-walls,occurat1.8Ga.
Somearethoughttohavebeenalgalcysts,whichformwhenalgaeturninto
inactiveballsduringdryperiods.Newlineagesalsoemerged,includingred
algaeat1.2Ga,whichwerelikelycapableofsexualreproduction.Today,we
boilredalgaedescendantstomakeagar,whichisusedtothickenicecream,and
weuseonetype,nori,towrapsushi.Becausesexwasdiscovered,theboring
billionwasnotthatboring.
Startingaround750Ma,andculminatingafterfitsandstartsabout580Ma,
atmosphericoxygenroseasecondtimetolevelsexceeding3percent,andthe
deepseabecamefullyoxygenated.Animalbiomarkersandtinyfossils(possibly
sponges)datefromaround630Ma.Butit’sonlyafter580Mathatwefind
complexfossilsfromcentimetrestometresinsize.Thesearestrangesoft-bodied
organismswithoutmouthsormusclesthatmusthavereliedupondiffusionof
nutrientsthroughtheirskin.Someresemblepizzaswhileotherslooklikeplant-
likefronds.However,theorganismslivedtoodeeplyintheseatoreceive
sunlightsotheycannothavebeenplants.Collectively,theyarecalledthe
EdiacaranbiotabecausetheywerefirstclearlyidentifiedintheEdiacaranHills
insouthernAustralia.TheEdiacaranslastedtensofmillionsofyearsbefore
dyingout.
Then,theCambrianExplosionoccurred,whichwastherelativelyrapid
appearanceofanimalfossilswithnewbodyarchitecturesina10-to30-million-
yearintervalafterthebeginningoftheCambrianPeriodat541Ma.
Representativesofmostmoderngroupsofanimalsemerged,includingmany
withhardskeletons.TheCambrian(541–488Ma)comesdirectlyaftertheendof
ProterozoicAeon,andisthefirstperiodofthePhanerozoicAeon(541Mato
present).Phanerozoicmeans‘visibleanimals’fromtheGreekphaneros(visible)
andzoion(animal).OnetinyCambriancreature,Pikaia,wasasortofswimming
worm,afewcentimetreslong.Pikaiawasprobablyinagroupfromwhichall
thevertebrates,includingus,evolved.
Thesecondriseofoxygenallowedadiversityofanimalstoarisebutitscause
remainsunsolved.Suggestionsincludeorganiccarbonburial(andhenceoxygen
production)acceleratedbyseveralfactors.Thesewerenewlifeonlandthat
boostedthebreakdownofsurfacesandnutrientrelease,theevolutionofmarine
zooplankton,andtheenhancedproductionofclaysthathelpedtoburyorganic
matter.Alternatively,moderatelevelsofaround100partspermillionof
atmosphericmethanethroughoutthe‘boringbillion’mighthavepromotedyet
morehydrogenescapetospacethatoxidizedtheseafloorandusheredinan
increaseofoxygenafterasecondglobalredoxtitration.
Around400Ma,athirdriseofoxygentookplacewhenvascularplants
colonizedthecontinents.Suchplantshavespecialtissues(suchaswood)to
transportwaterandminerals.Theyprobablyenhancednutrientrelease(by
breakingdownlandsurfaceswithroots)andorganicburial.Afterwards,
atmosphericoxygenlevelsremainedwithin10percentto30percent,allowing
thepersistenceofanimalsandeventuallyourownappearance.
SnowballEarthorWaterbeltEarth?
Curiously,worldwideglaciationsareassociatedwithboththeGreatOxidation
Eventandthesecondriseofoxygen,likebookendsencompassingthe‘boring
billion’.Somescientistsarguethatatthesetimes,theEarth’soceansentirely
icedoverinaso-calledSnowballEarth.Thisstatewasfarmoreextremethan
theiceagesthathaveoccurredinthelastfewmillionyearsthathaveonly
extendeddowntomid-latitudes,notthetropics.Forcontext,theProterozoic
aeonisdividedintothreeeras,thePalaeoproterozoic(2.5–1.6Ga),
Mesoproterozoic(1.6–1.0Ga),andNeoproterozoic(1.0–0.541Ga).Geologists
commonlyspeakofthe‘Palaeoproterozoicglaciations’todescribethreeorfour
glaciationsthatoccurredatthebeginning(2.45–2.22Ga)ofthe
Palaeoproterozoic,whilethe‘Neoproterozoicglaciations’referstotwolarge
glaciations(at715and635Ma)andonesmallone(at582Ma).
Therockrecordprovidesevidenceoftheseworldwideglaciations.Theflowof
icesheetsmixesstones,sand,andmudintoatypeofrockcalledtillite.Glaciers
alsoleaveparallelscratchmarkscalledstriationsonunderlyingrocks.
Furthermore,rocksdropoutoficebergsormeltingiceshelvesandendupbelow
asso-calleddropstonesthatdeformsedimentarybeds.Together,tillites,glacial
striations,anddropstonesaredeadgiveawaysthaticesheetswereoncepresent.
Magneticmeasurementsmadeinthe1980sand1990salsodemonstratedthatthe
glaciationsextendedintothetropics.AcompassshowstheEarth’snorth–south
magneticfield.Lessobviousisthefactthatthemagneticfieldlinesare
approximatelyperpendiculartothesurfacenearthepolesandparalleltothe
surfaceneartheequator.Volcanicrockscontainironminerals,suchasmagnetite
(anironoxide,Fe
3
O
4
),whichbecomemagnetizedinthedirectionofthe
prevailingmagneticfieldwhentheycool.Thus,whenthetrappedorremnant
magneticfieldlinesareparalleltoancientbedsofrock,weknowthattherocks
formedinthetropics.Thiswasdiscoveredinrocksassociatedwith
NeoproterozoicandPaleoproterozoicglaciations.
TheideathattheentireEarthmightfreezeoverifseaicereachedthetropics
originatedinthe1960s.ARussianclimatologist,MikhailBudyko,calculated
thatifpolaricecapsextendedfartherequatorwardthan30°latitude,theentire
Earthshouldfreeze.Icereflectsalotofsunlightandsohasahighalbedo,which
isthefraction(between0and1)reflected.Today,theEarth’salbedoisabout0.3,
meaningthat30percentofthesunlightisreflectedtospace.Butseaicereflects
50percentwhenbareor70percentifcoveredinsnow.Adisastrousice-albedo
runawayoccurswhenhigh-albedoiceinthetropicsmakestheEarthabsorbless
sunlightandcool,whichcreatesmoreicethatmakestheEarthevencolder,and
soon.Afrozenglobeiscreatedwithanaveragetemperaturebelow–35°Cand
anicethicknessof1.5kmattheequatorand3kmatthepoles.
JoeKirschvinkoftheCaliforniaInstituteofTechnologyproposedhowa
SnowballEarthwouldmelt,aswellastheveryphraseSnowballEarth.He
suggestedthatvolcanoeswouldpunchthroughtheiceandventcarbondioxide
intotheatmosphere.Becausetherewouldbenorainfallorsignificantopen
oceantodissolvecarbondioxide,itwouldbuildupoverafewmillionyears
untilanenhancedgreenhouseeffectmeltedtheSnowball.Also,theflowofice
fromthickpolaricetothintropicalicewouldcarryvolcanicashandwindblown
dust,sothatthetropicswouldbecomedarker,makingtheSnowballproneto
melt.Thentheice-albedorunawaywouldruninreverse:lessicewouldmakethe
Earthwarmer,whichwouldmeltmoreice,andsoon.
Layersofcarbonaterockcalledcapcarbonatesthatsitontopoftheglacial
depositspossiblyattesttothisaftermathofaSnowballEarth.Theideaisthat
aftertheicemelted,largeamountsofcarbondioxidewereconvertedintocap
carbonates.ProponentsoftheSnowballEarthhypothesisnotethattheratioof
carbon-12tocarbon-13isotopesinthecarbonateissimilartothatinvolcanic
carbondioxide.
AcentralproblemfortheSnowballEarthhypothesisishowphotosyntheticalgae
survivedunderthethickice,giventhattheyrequiresunlight.Lineagesofgreen
andredalgaewentstraightthroughtheNeoproterozoicsnowballsunscathed.
Onesuggestionisthatheataroundvolcanoesallowedsomeopenwaterorthin,
transparentice.ButothersquestionwhethertheentireEarthreallyfrozeover.
SomereasonablysophisticatedcomputersimulationsoftheEarth’sancient
climateproduceatropicalbeltofopenwater—aWaterbeltEarth—becausethis
regionissurroundedbybareiceofmoderatealbedo,whereastheveryreflective,
snow-coverediceoccursonlyathighlatitudes.
Whateverhappened,thecauseofboththeNeoproterozoicandPaleoproterozoic
glaciationswasprobablyadiminishedgreenhouseeffect.Continentsdrift
becauseofplatetectonics,andreconstructionshavethembunchedupinthe
tropicspriortotheancientsnowballeras.Rainfallovertropicalcontinents
perhapsdrewcarbondioxidedowntolowerlevelsthantypical,poisingtheEarth
forglaciation.Also,thecoincidenceofrisesinoxygenwiththeglaciationeras
suggestsatriggerthroughmethane,anothergreenhousegas.Probablythe
oxygenincreaseswereassociatedwithrapiddecreasesinmethanebecausehigh
abundancesofthetwogasesareincompatible—theyreact.
Forastrobiology,thelessonfromSnowballEarthisthatthebiosphereonan
Earth-likeplanetisresilient.Despitedrasticclimateswings,lifeonEarth
survivedandemergedaftertheNeoproterozoicintoanewaeonofanimals.
Ontheoccurrenceofadvancedlife
Inastrobiology,weoftenwonderifanimal-likelifeexistselsewhereinthe
galaxy.ThatpossibilitycanperhapsbeinformedbyevolutiononEarth.To
becomecomplex,terrestriallifehadtoovercomeatleasttwokeyhurdles.First,
lifehadtoacquirecomplexcells,whichwereneededfordifferentiationonthe
largerscale,i.e.cellswithfunctionaldistinctions,suchasliverversusbraincells.
Large,three-dimensionalmulticellularanimalsandplantsaremadeonlyof
eukaryoticcells,whichcandevelopintoamuchlargerrangeofcelltypes
comparedtothecellsofmicrobes(seeChapter5).
Asecondprecursorforanimalswashavingsufficientoxygentogeneratelotsof
energywithanaerobic(oxygen-using)metabolism,suchasourown.Abundant
energyisneededtogrowlargeandmove.Anaerobicmetabolism,whichdoesn’t
useoxygen,producesabouttentimeslessenergyforthesamefoodintakethan
aerobicmetabolism.Forextraterrestriallife,wemightwonderiffluorineor
chlorine—whicharepowerfuloxidants—couldbeusedinplaceofoxygento
generatehighenergylevels.Theanswerisno.Fluorineissoreactivethatit
explodeswithorganicmatter,whilechlorinedissolvestomakeharmfulbleach.
Oxygen,itappears,isthebestelixirforcomplexlifeintheperiodictable—
reactiveenoughbutnottooaggressive.
Oxygennotonlyneedstobepresentbutalsoconcentrated.Thefirstaerobic
multicellularlifeprobablyconsistedofaggregatesofcellsproducedwhen
dividingcellsfailedtoseparate.Suchagglomerationswouldhavebeenlimited
insizebydiffusionofoxygentotheinnermostcellsothatgreaterlevelsof
externaloxygenwouldhavepermittedlargercellgroupings.Attypical
metabolicrates,adiffusion-constrainedanimalprecursorofaboutafew
millimetres’dimensionwouldneedatmosphericoxygenconcentrations
exceeding3percent.Thiscorrespondstotheoxygenincreasearound580Ma
thatusheredintheanimals.
Fossilalgaeshowthateukaryotesexistedlongbeforeanimals,sotheslowfuse
totheCambrianExplosionwasprobablythetimeittooktobuildupadequate
oxygen.Wecanevendefineanoxygenationtimeasthatrequiredtoreach
oxygenlevelssufficientforcomplexanimallife.OnEarth,itwas4billionyears
aftertheplanetformed.
TheoxygenationtimeonEarth-likeexoplanetsisuncertaingiventhatwe’restill
tryingtounderstandwhatsetthetimescalefortheEarth.However,oxygen
comesfromliquidwater.SoEarth-likeexoplanetswithoceanshavethepotential
todevelopoxygen-richatmospheresifwater-splittingphotosynthesisevolves.If
theoxygenationtimeexceeds12billionyearsonacertainexoplanetbecauseit’s
endowedwithmaterialthatreactswithoxygen,aSun-likeparentstarwouldturn
intoa‘redgiant’beforetheconditionsforanimallifebecamepossible,andthe
planetwouldbedoomedtopossessnothingmorecomplexthanmicrobialslime.
Butifotherexoplanetshaveshortoxygenationtimes,complexlifemightoccur
fasterthanittookonEarth.
Trendsinevolution?Thelessonofmassextinctions
Onceanimalsevolve,wemightwonderifevolutionhasatrendtowardsgreater
complexity.Considerlifeonland.Inlandcolonization,thefirstplantsporesare
foundaround470Ma,whilefossilsofsubstantialpartsofplantsappearabout
425Ma.Insectsappearonlandaround400Ma.Thenfishevolvedinto
amphibiansaround365Maandtheirdescendantsultimatelybecamethereptiles
andmammals,includingus.Thisisatrendforthebiospheretousemoreofthe
Earth’sresources.Butamisconceptionistothinkofevolutionasasteady
progressionendinginus.Thefossilrecordshowsthatspeciescomeandgo.
Also,massextinctionsintermittentlyprunethediversityofcomplexlife.They
areeventswhenmorethan25percentoffamiliesarelost,where‘family’isthe
biologicaltierabovegenusandspecies(seeChapter5).Suchmassextinctions
refertothenon-microbialpartofthebiosphere,ofcourse.
OnlyaroundtheProterozoic–Phanerozoictransitionisthereadramaticincrease
indiversitybecauseatthattimeorganismsacquirednewbodyarchitecturesand
waysoflivingthatpersisted.Inanimals,akeyinnovationwasthebodycavity,
thecoelom(pronounced‘see-lum’),whichcouldbefilledwithfluidtoprovide
rigidityandallowtheconcentrationofforcesfrommuscles.Thisfacilitatedself-
propulsion,whichwasfurtherimprovedwithhardskeletons,atfirstexternaland
theninternal.Apredator–preyevolutionaryarmsracelikelycontributedtothe
CambrianExplosionofanimalvarieties.Plantdiversitylaterincreasedwiththe
developmentofvascularsystemswithrigidcellsfortransportandassociated
organsthateventuallygaverisetotrees.
Inthepast500Ma,therehavebeenfivemassextinctionsthatkilledover50per
centofspecies,withthetwobiggestat251Maand65Ma,respectively.
Excludingcrocodilians,nolandanimallargerthanthesizeofadomesticdoggot
throughthelargestevent,separatingthePermian(299–251Ma)andTriassic
(251–200Ma)periods.Thiseventtookamassivetollintheoceans.For
example,thetrilobites—iconsoftheCambrianseas—hadbeenindeclinebutthe
Permian–Triassicextinctionfinishedthemoff.Thesecondlargesteventat65
MawipedoutthedinosaursandseparatestheCretaceous(145–65Ma)and
Paleogene(65–23Ma)periods.(Someliteraturereferstothiseventas
Cretaceous-Tertiarybasedonanoldernamingsystem.)ThePermian–Triassic
massextinctiondestroyedasmuchas95percentofmarinespeciesand70–80
percentoflandanimals,whiletheCretaceous–Paleogeneextinguished65–75
percentofallspecies.
ThecauseofthePermian–Triassicmassextinctionwasapparentlyachainof
misfortunesgeneratedbytheEarthherself.Thetriggerseemstohavebeenlarge-
scalevolcanisminSiberia,coveringanareasimilartoEurope.Becausecoal
depositsunderlaythisarea,thevolcanismpumpedouthugeamountsofcoal-
derivedmethaneandcarbondioxidegreenhousegases,whichwarmedand
acidifiedtheocean.Oxygenislesssolubleinwarmwater,soananoxicdeepsea
developed,whichmayhavebelchedpoisonoushydrogensulphidetothesurface.
Thecombinationofclimaticwarming,oceanacidification,andnoxiousgases
extinguishedmorelifethananyothereventinthePhanerozoic.
Theimpactofanasteroidofabout10kilometresdiameterappearstohave
causedtheCreataceous–Paleogenemassextinction.Calamitousconsequences
includedtemporarydestructionoftheozonelayer,worldwidewildfires,acid
rain,andsubsequentclimaticcoolingcausedbysulphateparticlesinjectedinto
thestratospherethatreflectedsunlight.Theimpactcraterhasevenbeenfound
aboutakilometreundergroundneartheMexicantownofChicxulub
(pronounced‘sheek-soo-loob’).
Extinctionsdestroypreviouslysuccessfullineagesbuttheyalsoprovide
opportunitiesforothers.YouarereadingthisbookbecauseoftheChicxulub
impactor.Themammalsbecamedominantoncethedinosaursweregone.
Ontheotherhand,massextinctioneventsmeanthatifacivilizationdevelops,it
canbewipedout.ImpactsthesizeofthatproducingtheCretaceous–Paleogene
extinctionshouldoccurevery100millionyearsorsoonEarth,giveortakea
factorofafew.Theimplicationisthatcivilizationsonexoplanetscouldbeshort-
livedcomparedwiththeageoftheuniversemerelybecauseofsuchrandom
catastrophes.Thus,anEarth-likeplanetcanremainfitforlife,butcivilizations
areprobablyephemeral.Thelatterhasconsequencesintheastronomicalsearch
forextraterrestrialintelligence(Chapter7).
Chapter5
Life:agenome’swayofmakingmoreand
fittergenomes
LifeonEarth:theviewfromabove
InChapter1,Ipresentedageneraldefinitionoflife,butinfindinglife
elsewhere,ithelpstoknowouroneexampleoflifeingreatdetail.Tothisend,
let’sstartwithaglobalperspectiveofterrestrialbiologyandworkdownwardto
cellsandmolecules.
ImagineaninterstellartravellerwhoarrivesontheEarthandwantstoknow
aboutourbiology.Perhapsfromherplanet,shehaddeducedthatlifeexistshere
becauseofEarth’swet,anomalouslyoxygen-richatmosphere,orshehadpicked
upTVbroadcastsfromdecadesagothatsomehowdidn’tputheroffvisiting.
Amazingly,shespeaksEnglish(asextraterrestrialsalwaysdointhemovies)and
byastrokeofluckherspacecraftlandsinanEnglish-speakingcountry!What
wouldwetellher?
Atthegloballevel,Earth’sbiosphereisthesumofalllivinganddead
organisms.Sometimesthetermincludesthenon-livingregionsthatlife
occupies.Byquantifyingthebiosphereinbillionsoftonnesofcarbon(1tonne=
1,000kg),wecanidentifyitsbroadcomponents.Thebiomassonlandisaround
2,000billiontonnesofcarbonofwhich30–50percentislivingandtherestis
dead.Intheocean,only0.1–0.2percentofabout1,000billiontonnesof
biomasscarbonisalive.Forestsarethereasonthatthere’ssomuchmoreliving
biomassonlandthanintheoceans.
Anunsettledissuethat’srelevanttolifeelsewhere(belowthesurfaceofMarsor
Jupitersmoon,Europa)istheextentofEarth’ssubsurfacebiosphereor
‘intraterrestriallife.’Somescientistssuggestthatahugemassofmicrobes
extendsakilometreortwobelowtheseafloorandmorethan3kilometres
underneathland.Alimitforlifeatsuchdepthsistemperature.Asyougo
downwards,itgetswarmer(asminersknow)and,atsomepoint,toohotforeven
thetoughestmicrobes.Earth’ssubsurfacebiospherebiomassisuncertain
becausedeepdrillinghasnotbeendoneforalltypesofsubsurface
environments,butestimatesrangefromabout1percentto30percentofEarth’s
livingbiomass.
Whateveritsprecisemass,thebiosphereislessthanabillionthofthemassof
theEarthandyetmanagestogreatlyinfluencethechemistryofthesurface
environment.Thebiospherecandothisbecauseitprocessesvastamountsof
materialwithrapidturnover.Indoingso,individualsliveanddiealmost
instantlyongeologicaltimescales.Microbestypicallyreproduceintensof
minutestodays,whilelargemulticellularorganismslastonlyafewthousand
yearsatmostbeforetheybecomedeadfodderformicrobialdegradation.For
example,amongstthelatter,theoldestnon-clonalorganismisaBristleconePine
intheWhiteMountainsofCalifornia,whichgerminatedaround3049BC,several
centuriesbeforetheearliestEgyptianpyramids,butamereblipingeological
time.
Themainactivitiesintoday’sbiosphereareoxygenicphotosynthesisandits
chemicalreversalthroughaerobicrespirationandoxidation(Chapter4).
However,microbespossessahugerangeofothermetabolisms,whichwe
discusslater.
Microbes,ofcourse,arefoundalmosteverywhereattheEarth’ssurface.A
seawaterandfreshwaterbacteriumcalledPelagibacterubiqueisprobablythe
mostnumerousorganismonEarth.Despitethat,itwasfirstdescribedonlyin
2002,whichshowshowbiologyisstilldeveloping.Overall,thereareabout10
29
microbesofallvarietiesintheocean,farexceedingthe10
22
starsinthe
observableuniverse.Microbesarealsoabundantonland.Therearetypically
about100millionto10billionmicrobespergrammeoftopsoil.Indoorair
usuallyhasaboutamillionbacteriapercubicmetre.Anaverageofonemicrobe
(albeitdead)evenfloatsinevery55cubicmetresofairat32kilometresaltitude
inthestratosphere.
TherearefourkeypropertiesthathaveallowedtheEarthtobecomeso
extensivelyinhabited.Themostimportantiswidespreadliquidwater.All
metabolizingorganismscontainorganicmoleculesdispersedinaqueous
solution.Consequently,wedon’texpectlifetoexistonacompletelydrysurface,
suchasVenus.Thesecondessentialpropertyishavingenergyformetabolism.
SunlightisthemainsourceforEarth’sbiosphere,butsomeorganismsobtain
energyfromchemicalreactionsindarkness,whichmeansthatit’snotimpossible
thatmicrobial-likelifemightexistbelowthesurfaceofMarsorEuropa.Athird
life-givingfeatureisarenewablesupplyofessentialchemicalelements.Aplanet
thatcan’tresupplyvitalelementsthroughnaturalcycles(suchasthewatercycle
ortectonics)wouldbedeathly.Afourthattribute,whichmaybeessentialfor
life,isthepresenceofinterfacesbetweensolids,liquids,andgases.It’s
advantageoustoliveatastableinterface,suchasonlandorthesurfaceofthe
ocean.ThisiswhyitwouldbedifficultforlifetoexistonagasgiantlikeJupiter
thathasnosurface.Lifemightspeculativelybepossibleatcertainaltitudesin
Jupitersatmospherebutdeepchurningbyconvectionwouldperiodicallyplunge
lifeintoaninterioroffatalheatandpressure.
Aninsideviewofterrestriallife:thecell
Wehavejustconsideredbiologyonaglobalscalebutunderthemicroscopeall
organismsarecomposedofcellsofdifferenttypesthatputthemintooneofthree
domains,theEukarya,Archaea,orBacteria.Thelasttwoaremicrobialandare
sometimeslumpedtogetherasprokaryotes,althoughmanymicrobiologistsnow
considerthistermantiquatedbecausearchaeaandbacteriaarebiochemically
dissimilar.DNAfloatsfreelyinthemiddleofthearchaealandbacterialcells,
whereasineukaryotestheDNAishousedinsideamembrane-boundnucleus.
Archaeaandbacteriaaresinglecells,withtheexceptionofsomebacterial
speciesthatjoinupinarowtoformfilaments.Eukaryotescanbesinglecelled,
suchasanamoebaorabakersyeast,butonlyeukaryotesformlarge,three-
dimensionalmulticellularorganisms,suchasmushroomsorhumans.
Theclassificationintothreedomainsoflifewasmotivatedbygeneticsand
supersedesanolder‘fivekingdom’systemofplants,animals,fungi,protists
(single-celledeukaryotes),andbacteria.However,theseoldtermsarestillused
intaxonomy,whichclassifiesanorganismbelowitsdomainaccordingto
Kingdom,Phylum,Class,Order,Family,Genus,andSpecies.ThemnemonicI
usetoremembertheselevels(ortaxa)isfartoorudetomentionbutanotheris
‘KeepingPreciousCreaturesOrganizedForGrumpyScientists’.Thetaxonomic
levelsofahuman,forexample,aretheanimalkingdom,chordatephylum,
mammalclass,primateorder,hominidfamily,Homogenus,andsapiensspecies.
TheSwedishbotanistCarolusLinnaeus(1707–78),whogaveustheword
biology,developedmoderntaxonomy,includingbinomialnamesfororganisms,
e.g.Homosapiens.Butittooktheadventofevolutionarytheoryandmolecular
biologytouncoverthebiochemicalunityoflifeandgeneticcommonancestry.
Whiletherearesomesimilaritiesbetweeneukaryotesandtheothertwodomains,
there’salsoagulfincomplexity.Bacteriaandarchaeaareusuallyaround0.2–5
microns(millionthsofametre)insize,withrareexceptions,whereaseukaryotic
cellsaregenerallybigger,at10–100micronssize.Thelargereukaryoticcells
containorganellestoperformspecializedfunctions,analogoustotheorgansof
thehumanbody(Fig.4).Forexample,themitochondriacarryoutrespiration.In
plantoralgalcells,chloroplastsperformphotosynthesis.However,onefeature
commontoallcellsisalargenumberofribosomes,whichareglobular
structuresthatmakeproteins.Forexample,inprokaryotesorsimpleeukaryotes
suchasyeast,thecellmighthaveseveralthousandribosomes,whileinan
animalcellthenumbermightreachseveralmillion.
4.a)Schematicofprokaryote(archaeaandbacteria)versuseukaryote
structure;b)Twobacteriacaughtintheactofconjugation
Givenanastrobiologicalinterestincomplexextraterrestriallife,wemightask
whyonlytheeukaryoticcellproduceslarge,three-dimensionalmulticellularlife.
Theanswerisnotfullyknownbuteukaryoticcellshaveamoresophisticated
internaldynamiccellskeletonorcytoskeletonthanarchaeaorbacteria.This
consistsofproteinmicrofilaments,tinyproteintubes(‘microtubules’),and
molecularmotorsthatcontrolcellstructureandhelptransportsignalling
moleculestochangecellphysiology.Sotheabilitytodevelopintomany
specializedforms(forexample,skinorbraincells)isinherentinthemake-upof
aeukaryoticcell.Cyanobacteriaareabletomakefilamentsofhundredsofcells
inarowwithsomedifferentcelltypesbutthatisthelimit.Unlikearchaeaor
bacteria,eukaryoteshavebiggerandmoremodulargenomes,whichalsoallows
formorecomplexity.Iftherewerenoeukaryoticcells,theEarthwouldbemuch
duller.Allofthefamiliarorganismsofourworld—theanimals,plants,andfungi
—wouldn’texist.Thus,whenwethinkaboutcomplexlifeonexoplanets,we
shouldwonderwhetherevolutionwouldmakecellslikeeukaryoteselsewhere.
Forthisreason,theoriginofeukaryotesisofgreatinterest.
Moderngeneticsimpliesthattheeukaryoticcellisa‘Frankenstein’smonster’,
assembledinevolutionaryhistoryfrombitsandpiecesofbacteriaandarchaea.
Forexample,themitochondrionineukaryoticcellswasderivedfroma
bacteriumoriginallylivingsymbioticallyinsideanothercell.Thelargercell,
whichmayhavebeenanarchaeonorsome‘proto-eukaryote’thatnolonger
exists,swallowedthefree-livingbacteriumand,inthemostimportantgulpin
history,themitochondrialancestorcameintobeing.Indeed,separateDNAin
mitochondriaprovidesevidenceofbacterialancestry.ThedistinctDNAof
chloroplastsinplantandalgalcellsshowsthatchloroplastswerederivedina
similarwayfromsymbioticcyanobacteriathatendeduplivinginsidelarger
cells.Effectively,allthecellsintheleavesofgreenplantscontainancestorsof
cyanobacterialslavesthatwerecagedandco-optedlongago.Thetheoryofsuch
anoriginforthemitochondria,chloroplasts,andotherorganellesineukaryotic
cellsiscalledendosymbiosis.
Aworldwithouteukaryoteswouldalsobeonewithoutsex.Thinknoflowersor
lovesongs.Archaeaandbacteriaarenotsexedbuttheydoconjugatewhentwo
cellsareconnectedbyatubeinwhichgenesaretransferredinpiecesofDNA
calledplasmids.Bacterialsurfaceshaveprotuberancescalledpili,andduring
conjugationaspecialpilusextendstoapartner,providingtheconduit(Fig.4).
Unlikesexualreproductionineukaryotes,microbialconjugationdoesn’tproduce
offspringandisquickandeasy.It’sasifyoubrushedupagainstapersonwith
redhairinacoffeeshopandacquiredthered-hairedgenewithaninstantchange
ofyourhaircolour.Archaeaandbacteriacanalsoacquirenewgenesthrough
transformation(uptakeofforeignDNAfromtheenvironment)andtransduction
(virus-mediatedgeneswapping).Indeed,therapidacquisitionofgenesallows
thequickdevelopmentofbacterialresistancetoantibiotics.
Eukaryotesthataresexedgenerategametes,i.e.spermandeggcells.Incomplex
multicellulareukaryotes,thediversityoflifemakesitsurprisinglyhardtodefine
‘male’and‘female’,leavinguswiththeodddefinitionthatmalesarethosethat
producethesmallgametes,whilefemalesproducethebigones.Thegametes
fusesothathalfofthegenescomefromafatherandhalffromamother.Usually
DNAisalooselycoiledthreadbutbeforecelldivision,DNAcurlsupinto
visiblechromosomesunderamicroscope.Forexample,humanshave46
chromosomesin23pairsineachcell,exceptforthegametes,whichhavehalf,
i.e.23chromosomes.
Therearemanyideasaboutwhysexisevolutionarilyadvantageousfor
eukaryotes.Onepossibilityconcernshowitmixesandmatchesgenesfromboth
parentsontoeachchromosomeinaprocesscalledrecombination.Ifbeneficial
mutationsoccurseparatelyintwoindividuals,themixtureofbothcan’tbe
achievedinasexualorganisms,butsexuallyreproducingorganismscanbring
themtogetherandreapthebenefits.Conversely,sexcanalsoeliminatebad,
mutatedgenesbybringingunmutatedgenestogetherinsomeindividuals,
whereasself-cloningorganismsarestuckwithbadgenes,andoffspringcandie
becauseofthem.
Outsidethethreedomains,virusesrepresentagreyareabetweenthelivingand
non-living.Virusesaretypicallyabouttentimesmoreabundantthanmicrobesin
seawaterorsoil.TheyconsistofpiecesofDNAorRNAsurroundedbyprotein
and,insomecases,afurthermembrane.Virusesaretiny,onlyabout50–450
nanometres(billionthsofametre)insize,comparabletothewavelengthof
ultravioletlight.Theyaregenerallyconsiderednon-livingbecausetheyare
inanimateoutsideacellandhavetoinfectandhijackcellsfortheirown
reproduction.However,somedothiswithoutthehostevernoticing,sonotall
virusescausedisease.Onetheoryofseveralfortheoriginofthenucleusof
eukaryotesisthatitmayhaveevolvedfromalargeDNAvirus,buttheroleof
virusesintheevolutionoflifeisstillamatterofdebate.
Thechemistryoflife
Todiscussmanyaspectsoflife,suchasgeneticsandmetabolism,requiresthe
vocabularyofbiochemistry.Thefourmainclassesofbiomoleculearenucleic
acids,carbohydrates,proteins,andlipids.Likeself-assemblyfurniture,many
biomoleculesaremodular.Theyarechainsorpolymersofsmallerunitscalled
monomers.
Carbohydratesprovideenergyandstructure.TheycontainC,H,andOatomsin
a1:2:1ratio,withrepeatedunitsofC(H
2
O),soliterally,theirchemical
compositionis‘carbonhydrated’.Sugarswithfivecarbonatomsarefoundin
DNAandRNAmolecules,whilesix-carbonsugarsexistincellwalls,suchas
celluloseinplants.
Lipidsareorganicmoleculesthatareinsolubleinwaterbutdissolveinanon-
polarorganicsolvent—onewithoutsignificantelectricalchargeonanyofits
atoms,suchasoliveoil.Thus,ifwegroundupadeadanimalandbatheditina
non-polarsolvent,anythingthatdissolvedwouldbealipid.Lifeuseslipidsin
cellmembranes,asfatsforenergystorage,andassignallingmolecules.Major
componentsofmembranesarephospholipids,whichhaveahydrophilic(water-
loving)endthatcontainsphosphorusandahydrophobic(water-repellent)tail
thatisahydrocarbon,consistingofcarbonandhydrogenatoms.Adoublelayer
ofphospholipids,calledabilayer,formsamembrane.Thehydrophilicendsstick
outintoanaqueousmediumontheinteriorandexteriorofacell,while
hydrophobictailsfaceeachotherinthemiddleofthemembrane.
Proteinsarepolymersmadeupofaminoacidunits.Theiruseincludesenzymes
andstructuralmolecules,althoughthereisalonglistofotherfunctions.
RNAandDNAarenucleicacids,whicharepolymersofnucleotidemonomers.
Eachnucleotideismadeupofafive-carbonsugar,aphosphate,andapartcalled
abase(Fig.5).InDNA,therearefourpossiblebases.Allofthemcontainoneor
tworingsofsixatomswherefouroftheatomsarecarbonandtwoarenitrogen.
EachbasehasaletterdesignationofA,C,G,andT,whichstandforadenine,
cytosine,guanine,andthyminemolecules,respectively.MoleculesofRNAuse
thesamethreebasesexceptthattheTofDNAisreplacedbyaUforuracil.
5.Left:DNAconsistsoftwostrandsconnectedtogether.Eachstrandis
madeupofa‘backbone’ofphosphate(P)andsugar(S)components.The
strandsconnecttoeachotherwithbasepairs.Right:Inthreedimensions,
eachstrandisahelix,sothatoverallwehavea‘doublehelix’
In1953,JamesWatsonandFrancisCrickfamouslydeducedthestructureof
DNA:twopolynucleotidestrandscoiledinascrew-likehelix(Fig.5).Thebases
atthesidesofeachstrandsticktogetherby‘hydrogenbonds’inwhichaslightly
positivelychargedhydrogenatomononebaseisattractedtoaslightly
negativelychargedatomonabasefromtheoppositestrand.Structural
compatibilityonlyallowsaCbasetopairwithGandanAtopairwithT.Each
DNAmoleculehasseveralmillionnucleotides,andeachchromosomeinthecell
containsaDNAdoublehelix.Incontrast,RNAmoleculesaremostlysingle
stranded.However,RNAcanfoldbackonitselfifcomplementarybasesexistin
twoseparatepartsofthestrand,notingthatadeninepairswithuracil(U)in
RNA.
ThestructureofDNAincorporatestwofundamentalcharacteristicsoflife
identifiedinChapter1:anabilitytoreproduce;andablueprintfordevelopment
andmaintenance.Inreplication,theDNAhelixsplitsintotwostrandswitheach
servingasatemplateforanewcomplementarystrand.Forexample,whereverA
appearsonthetemplate,aTisaddedtothenewlygeneratedstrand,orvice
versa.ThesameappliesforG–Cpairs.Intheprocess,mutationsandmistakes
allowforevolution.
Infulfillingitsotherroleastheblueprint,theDNAdoublehelixisunzippedby
anenzymetoprovideinstructionstogenerateproteins.Partoftheunzipped
DNAundergoestranscriptionintoastrandofmessengerRNA(mRNA),which
isacomplementarycopyoftheDNA,exceptthatU(insteadofT)isinserted
whereverAappearsintheDNA.ThenthemRNAisfedintoaribosomelikea
ribbon.Inthegeneticcode,groupsofthreeletters(calledcodons)alongthe
mRNAspecifyeachaminoacidinaproteinthatflowsoutoftheribosome.
Apartfromreproduction,lifehastosustainitselfthroughmetabolism,which
involvesbreakingdownmoleculestomakeenergy(catabolism)aswellas
buildingupbiomolecules(anabolism).Theclassificationofmetabolisms
dependsontheneedforenergyandcarbon.Becauseeachoftheserequirements
can,inturn,besatisfiedintwodifferentways,biologistshave2×2=4
metabolictermsfororganisms:chemoheterotroph,chemoautotroph,
photoheterotroph,andphotoautotroph(Fig.6).
6.Theclassificationschemeformetabolismsinterrestriallife
Allorganisms(probablyevenextraterrestrialones)fallintooneormoreofthese
categories.Thetrophsuffixmeans‘tofeed’andthetwowaysinwhich
organismsgetenergy,fromchemicalsourcesorsunlight,giverisetothechemo-
andphoto-prefixes.Anadditionalhetero-orauto-prefixisemployeddepending
uponthemethodusedtoacquirecarbon.Ifcarbonisobtainedbyconsuming
organiccarboncompounds(suchassugars),thehetero-prefixapplies.Ifan
organismconvertsinorganiccarbon(e.g.carbondioxide)intoorganiccarbon—
called‘fixingcarbon’—theauto-prefixisused.Ingeneral,heterotrophsmust
acquirefoodtomakeenergy,whileautotrophscanfixcarbonandmaketheir
ownenergy.
Wehumansarechemoheterotrophs.YouandIconsumeorganicchemicalsmade
byotherorganisms,suchasplants.Allanimalsandfungi,manyprotists,and
mostknownmicrobesarechemoheterotrophs.Incontrast,achemoautotrophisa
microbethatusesinorganicchemicalssuchashydrogen,hydrogensulphide,
iron,orammoniatomakeenergy,someofwhichitusestoextractcarbonfrom
carbondioxide.Forexample,chemoautotrophicmicrobesliveindarknessin
deep-seahydrothermalventsbyoxidizinghydrogensulphideandother
substances.(Chemoautotrophsarealsocalledchemolithotrophs,fromtheGreek
‘lithos’forstonebecausetheinorganicchemicalstheyusecomefromgeological
sources.)Plants,algae,andsomecyanobacteriaareallphotoautotrophsbecause
theyusesunlightforenergyandacquirecarbonfromtheair.Photosynthetic
bacteriafromtheChloroflexusgenus,whicharefoundinhotsprings,arean
exampleofmicrobesthatmetabolizeasphotoheterotrophsusingsunlightfor
energyandacquiringcarbonfromorganiccompoundsmadebyothermicrobes.
ChemoautotrophsmightinhabitthesubsurfaceofMarsorEuropa,while
phototrophsmightexistintheoceansofhabitableexoplanets.
Thetree(orweb)oflife
ThehistoryoflifeonEarthcanbededucedfromthewaythatevolutionhas
alteredgenes.Evolutionisthechangeininheritedcharacteristicsinapopulation
fromonegenerationtothenext.Becauseindividualsaregeneticallyvariable,in
anygivenenvironmentsomewillbebetteradaptedandhavegreater
reproductivesuccessthanothers,whichbiologistsdescribeashigherfitness.In
everygeneration,individualsoflowerfitnessarelost.Thisisnaturalselection.
So,overmanygenerations,lineagesaccumulategeneticadaptationsandnew
speciesevolve.
Insexualorganisms,speciesaregroupsthatcannotinterbreedundernatural
conditions,suchashumansandhorses.Manybacteriaandarchaealivein
differentecologicalnichesthatdefineseparategroups,buttheeasewithwhich
genescanmovebetweenmicrobesmakesitmoredifficulttodesignatespecies
ofmicrobe.Consequently,geneticsisused.Adifferenceofabout2–3percentin
thegenesthatcodetheRNAcontainedinribosomesisenoughtoseparate
microbialspecies.
7.The‘treeoflife’constructedfromribosomalRNA
Infact,wecanassesstherelatednessofalllifeformsbycomparinggenes.For
example,genesindicatethatyouandthefungusbetweenyourtoes—whichare
botheukaryotes—aremuchcloserrelativesthananarchaeonandabacterium,
despiteappearances.Fromthemid1960sonwards,varioustechniqueshavebeen
developedtoassessorganismsatthemolecularlevel,includingacomparisonof
thesequencesofeitheraminoacidsinproteinsornucleotidesinRNAandDNA.
Genesdefinethesequenceofaminoacidsinaprotein,sodifferencesinthe
‘proteinsequence’betweenspeciesindicatedisparityorrelatedness.For
example,‘cytochromec’isarespiratoryproteinof104aminoacidsfoundin
manyorganisms.Thesequencesareidenticalinhumansandchimps,showing
theircloserelationship.Comparedtothehumanprotein,arhesusmonkeyhas
onedifferentaminoacid,adoghasthirteendifferentaminoacids,andsoon.As
speciesbecomemoredistantlyrelated,theproteinsequencesdiverge.Withthis
sortofdata,ananalystcandrawupatree,likeafamilytree,thatrelatesallthe
species.
Inthe1970s,theAmericanmicrobiologistCarlWoese(1928–2012)usednucleic
acidsratherthanproteinstodeterminerelationshipsamongorganisms.He
examinedRNAinribosomes(rRNA,forshort),withthekeyinsightthatthis
enabledanalysesofalllife.Proteinsynthesis,thejobofribosomes,isacore
functionofanycell.Consequently,thegenesthatcodeforrRNAshouldhave
mutatedslowlyovertimebecausemostmutationsforthisprocesswouldhave
beenfatalandsofailedtoaccumulate.About60percentofthedryweightof
ribosomesismadeupofrRNA,andtheremainderisprotein.Ribosomesin
prokaryotesaresmallerthanineukaryotes,butotherwisesimilarinstructureand
function,allowingtheircomparison.
WoeseisolatedasmallrRNAandbycomparingthedifferencesinthenucleotide
sequences,hebuilta‘treeoflife’(Fig.7).Theconstructionoftheevolutionary
historyoforganismsinthiswayiscalledphylogeny.Woese’streewasashock
becauselifegroupedintothethreedomainsmentionedearlier,unlikethe‘five
kingdom’paradigminwhicharchaeawerelumpedwiththebacteria.
Furthermore,Woese’streedemotedplants,animals,andfungitomeretwigsat
theendoftheeukaryoticbranch.
Today,whileWoese’sconceptionofthreedomainsiswidelyaccepted,studiesof
othergenesshowthatatreewithverticaldescentfromonegenerationtothenext
istoosimple.Thisisbecausemicrobesswapgeneswilly-nilly(Fig.4),
sometimestounrelatedspecies,whichiscalledlateral(orhorizontal)gene
transfer.Thus,microbialbranchesofthetreeoflifearemorelikea‘weboflife’,
criss-crossedbylateralgenetransfers.
Sincethemid1990s,DNAsequencinghasbecomemoreautomated.For
sequencingasinglegenefromenvironmentalsamples,theprocessisasfollows:
isolateDNAfromcells→copy(or‘amplify’)aDNAgenemanytimesusinga
procedurecalledthe‘polymerasechainreaction’→obtainthegenesequence
→comparewithotherorganisms→produceaphylogenetictree.Increasingly,
wholegenomesaresequenced,includingthehumangenome,whichcontains
roughly21,000protein-codinggenes.
Astonishingly,humanprotein-encodinggenescoveronly1.5percentof3
billionnucleotides.Therestofthesequenceissaidtobe‘non-coding’or‘junk
DNA’.Thesetermsareactuallymisnomersbecausemanypartsofnon-coding
DNAregulatewhencertaingenesareexpressed(i.e.giverisetoproteins)or
codefornon-proteinproductssuchasrRNAs.
Thesurprisingdivisionbetweenarchaeaandbacteriarevealedbygenesis
confirmedbybiochemicaldifferences.Forexample,bacterialcellwallscontain
peptidoglycan,whichconsistsofcarbohydraterodscross-linkedbyproteins,
whereasarchaealcellwallshavevariablechemistryofproteinorcarbohydrate,
orboth.Eukaryotecellwalls,forcomparison,aremadeofcelluloseinplants,
chitininfungi,andarenon-existentinanimalcells,whichhaveonlya
membrane.Furthermore,thecellmembranelipidsofbacteriaandarchaeadiffer.
First,therearedissimilarchemicallinkagesbetweenthehydrophobicand
hydrophilicendsofphospholipids.Second,insteadofalipidbilayerwith
hydrophobicendsdanglingface-to-faceinthemiddleofthemembrane,archaea
havemoleculesthatconnectallthewaythrough.Thisprovidesstrengthandis
onereasonwhysomearchaeacanliveinveryhotwaterashyperthermophiles.
Therearealsoecologicaldistinctions.Forexample,noarchaeonisapathogen,
soyou’llnevergetadiseasefromarchaeawhereasyoucanfromvarious
bacteria.
Onefurtheruseofphylogenyisthatthegeneticdifferencebetweentaxacanbe
relatedtothetimeingeologicalhistorythattheydiverged,makingamolecular
clock.Inthe1960s,EmileZuckerkandlandtheNobelPrize-winningchemist
LinusPaulingcomparedproteinsfortaxathatwereknownfromfossilstohave
divergedfromacommonancestor.Theyfoundthatthenumberofaminoacid
differencesisproportionaltoelapsedtime.Oneinterpretationisthatmost
changesare‘neutralmutations’thathavenoeffectonfitness.Similarly,the
numberofnucleotidesubstitutionsincertainDNAsequencesisproportionalto
elapsedtime.Molecularclocksworkbestwithcloselyrelatedgroupsofspecies
thatarelikelytohavehadsimilarratesofmutation.Distantlyrelatedspecies
havedisparategenerationtimesandmetabolicrateswithvariablemutationrates
thatmustbetakenintoaccount.Touseamolecularclock,acalibrationpoint
fromthefossilrecordfixesthedateofaparticularancestorinacomputer
algorithmappliedtothemoleculardata.Aswegobackverydeepintotime,
therearefewerfossils,sothetechniquebecomeschallenging.Nonetheless,
molecularclocksindicatethatthelastcommonancestorofanimalsoccurred
about750–800Ma,whichiscuriousbecauseitpredatestheoldestanimal
fossils.
Despitethecomplicationsoflateralgenetransfers,thetreeoflifeprovides
informationaboutearlylife.Thermophiles(microbesthatthriveathigh
temperatures)thatgrowbestat80–110°CarefoundneartherootoftherRNA
treeinthearchaeaandbacteria(Fig.7).Areasonableinferenceisthatthelast
commonancestorlivedinahydrothermalenvironment.Organismsclosetothe
rootarealsochemoautotrophs,suggestingthatprimitivemicrobesprobably
gainedenergyfrominorganiccompounds.Thetreealsoshowsthatcomplex
organisms—theplants,fungi,andanimals—werelatetoevolve,consistentwith
thefossilrecord.Consequently,ifEarth’sphylogenyisaguideforastrobiology,
theimplicationisthesameasthefossilrecord:lifeelsewhereoughttobemostly
microbialformuchofthehistoryofthehostplanet.
Lifeinextremeenvironments
Apartfrombeingclusterednearthelastcommonancestor,thermophileswere
thestartingpointforresearchintoextremophiles.Extremophilesareorganisms
thatthriveunderenvironmentalconditionsthatareextremefromahuman
perspective.In1965,ThomasBrock,anAmericanmicrobiologist,discovered
pinkfilamentsofbacterialivingattemperaturesof82–88°Cinasteamyhot
springinYellowstoneNationalPark.Atthattime,nolifewasknowntoexist
above73°C,soBrock’sdiscoveryspurredaninterestinexploringthelimitsof
life.
Althoughitwasn’tanticipated,Brock’seffortsalsoultimatelyenabledthe
explosioningenetics.Brockfoundanewbacterium,Thermusaquaticus,in
anotherhotspring.Fromthismicrobe,industrialscientistsisolatedanenzyme
stableathightemperaturesthatwasabletocatalysethepolymerasechain
reaction(PCR)—theDNAduplicationtechniquethatrevolutionizedbiology.
Thus,purescience—inthiscase,whatwenowcallastrobiology—endedup
benefitingsocietyunexpectedly.Today,PCRtechnologyisamultibillion-dollar
industry.Brock,however,gaveawayhisbacteriumanddidn’tgetapenny.
Often,thegrowth(meaningreplication)ofextremophileseitherrequiresextreme
conditionsorisoptimalatthem.Fortemperature,theupperlimitforeukaryotes
is62°C,comparedto95°Cforbacteriaand122°Cforarchaea.Therecord
holder,themethane-producingarchaeon,ormethanogen,Methanopyrus
kandleri,hasoptimalgrowthat98°C.
Identificationofvariousextremophiles(Box1)indicatesthatlifeexistsina
muchwiderrangeofenvironmentsthananyonethoughtfeasiblefiftyyearsago,
whichopensuppossibilitiesforlifebeyondEarth.Forexample,thermophiles
mightsurvivedeepundergroundonMarsbecausetheyexistdeepintheEarth’s
crust.AnotherexampleconcernslifeinLakeVostok(250kmlong,50kmwide,
and1.2kmdeep),whichsitsbelow4kilometresoficeineastAntarctica.Ice
some100metresabovethelakeisthoughttohavefrozenfromlakewater.
Oddly,itcontainstracesoftheDNAofchemoautotrophicthermophiles.This
suggeststhatbeneaththecoldlake(at–2°Cbecauseofthehighpressure),there
maybehydrothermalwatercontainingthermophilesemanatingfromfractures.
ThetechniqueshonedfordetectinglifeinlakessuchasVostokcanbeappliedin
thesearchforlifeelsewhereintheSolarSystem.Samplingforlifeinthe
subsurfaceoceanofJupitersmoon,Europa,involvessimilarchallenges.
Box1Extremophiles
Acidophiles:requireanacidicmediumatpH3orbelowforgrowth;some
tolerateapHofbelow0.
Alkaliphiles:requireanalkalinemediumabovepH9foroptimalgrowthand
someliveuptopH12.
Barophiles(orpiezophiles):liveoptimallyathighpressure;someliveatover
1,000timestheEarth’satmosphericpressure.
Endoliths:liveinsidetheporespaceofarock(endo=within,lithos=stone).
Halophiles:requireasaltymedium,atleastathirdassaltyasseawater(halo=
salt).
Hypoliths:liveunderstones(hypo=under).
Psychrophiles:growbestbelow15°C,whilesomegrowdownto–15°C,with
reportsaslowas–35°Cformetabolism.
Radioresistantmicrobes:resistionizingradiation,suchasfromradioactive
materials.
Thermophiles:thriveattemperaturesbetween60and80°C,whilea
hyperthemophilehasoptimalgrowthabove80°C;theycanbecontrastedwith
mesophiles,suchashumans,whichlivebetween15and50°C.
Xerophiles:growwithverylittleavailablewater,andmaybehalophilesor
endoliths(xeros=dry).
Chapter6
LifeintheSolarSystem
Whichworldsmightbehabitabletoday?
In2002,whileteachingastrobiology,Iofferedaprizetoanystudentwhocould
guessninecelestialbodiesuptotheorbitofPlutothatIreckonedmightpossibly
harbourextraterrestriallife(Table1).Noonewon.But,withthegrowthof
astrobiologydiscoveriesandonlineinformation,someonereceivedtheprizein
2010.TodayImightaddseveralmorebodies,butwe’llleavethoseuntiltheend
ofthisChapter.
ThetypeoflifethatI’mconsideringinTable1issimple,comparableto
microbes,andtheguidingprincipleconcernsliquidwater.OnEarth,wherever
wefindliquidwater,wefindlife,whetherinbubblinghotsprings,dropsofbrine
insideice,orfilmsofwateraroundmineralsdeepinthecrust.
Somegofurtherandspeculatethatweirdlife—organismsthatdon’tdependon
water—mightexistinlakesonTitan,thelargestmoonofSaturn.Titan’slakes
containliquidhydrocarbons,notwater,somewhatlikesmallseasofpetroleum.
Lifethatuseshydrocarbonsolventisunknown,andtherearesomearguments
fromphysicalchemistry,whichI’llmentionlater,thatsuchlifemightbe
difficult.Incontrast,there’snoquestionthatliquidwatercansupportlife.
Table1.NineabodeswherelifemightexistintheSolarSystemtoday.The
distancetotheSunisinAstronomicalUnits(AU),where1AUistheEarth–
Sundistanceofabout150millionkilometresor93millionmiles
Body Typeofbodyandaverage
distancefromtheSun
Whyitmighthave
life
Mars planet,1.5AU mighthavesubsurface
pocketsofliquidwater
Ceres largestasteroid,2.8AU mighthavea
subsurfaceocean
Europa,Ganymede,
Callisto
largeicymoonsofJupiter,5.2
AU
evidencefor
subsurfaceoceans
Enceladus icymoonofSaturn,9.8AU evidencefora
subsurfaceoceanorsea
andpresenceof
organics
Titan largestmoonofSaturn,9.8AU evidencefora
subsurfaceoceanand
presenceoforganics
Triton largestmoonofNeptune,30.1
AU
mighthavea
subsurfaceocean
Pluto largeKuiperBeltobject,39.3
AU
mighthavea
subsurfaceocean
Sunlightandthehabitabilityoftheinnerplanets
Whethertheinnerplanets—Mercury,Venus,Earth,orMars—haveliquidwater
mostlyhastodowithtemperatureand,thus,distancefromtheSun(Table2).
‘Location,location,location’doesn’tjustsellhousesbutiscriticalforplanetary
habitability.
Table2.Theinnerplanetsandfactorsthataffecttheircurrenthabitability
Locationmattersbecausesunlightspreadsintoaspherewithasurfaceareathat
growswiththesquareoftheplanet–Sundistance.Solarfluxisthewattage(like
alightbulb)ofsunlightreceivedpersquaremetre.AttheEarth’sorbitaldistance
of1AstronomicalUnit(AU),sunshineprovides1,366Wattsovereverysquare
metre,theequivalentofalmostfourteen100-Wattlightbulbs.Atthe5AU
distanceofJupiter,thesolarfluxisafactorof25timessmallerbecausethesame
energyspreadsoveraspherethathas5×5=25timesthearea.ForMars,at1.5
AU,thesolarfluxis2.25(=1.5×1.5)timessmallerthanforEarth.Incontrast,
Venus,at0.72AU,receivessolarfluxalmosttwiceasbigasthatoftheEarth.
Mercury,atonly0.4AUfromtheSun,islifeless.It’sthesmallestoftheeight
planets(abouttwo-fifthsthediameterofEarth)andhasnoliquidwaterand
probablyneverdid.Today,Mercury’sbarrensurfacesometimesreaches430°C.
IfMercuryoncehadanatmosphere,itwouldhaveburnedoffwhenMercury
formedbecauseoflowgravityandintensesunlight.
ThedistancefromtheSunexplainswhyVenusandMarsarehostiletolife,
althoughthat’snotthewholestory.Theroughly460°CsurfaceofVenusiseven
hotterthanMercurygets,whileMarsisanicydesertwithanaverage
temperatureof–56°C.VenusiscomparableinsizetotheEarth,butMarsis
smaller—aroundhalfthediameterandone-ninththemassofEarth.Infact,
Mars’ssmallsizeledtoitspoorhabitabilitytoday,aswe’llseelater.
Althoughsolarfluxisonefactor,thegreenhouseeffectonMarsandVenusalso
determinessurfacetemperatures.TheMartianatmosphereexertsonly0.006bar
surfacepressure,comparedtoEarth’s1bar.Thiswispyairisanaridmixof95.3
percentcarbondioxide,2.7percentnitrogen,andminorgases.Recallthat
Earth’sgreenhouseeffectis33°C.Becauseofatmosphericthinnessanddryness,
Mars’sgreenhouseeffectisonly7°C,whichleavestheplanetfrozen.Thetwo
maingasesinVenus’satmosphere,96.5percentcarbondioxideand3.5percent
nitrogen,havesimilarproportionstothoseonMars.Butinstarkcontrast,
Venus’satmosphereismassivelythickandhasahugepressureof93baratan
averagegroundelevation.Asaresult,Venus’sgreenhouseeffectisawhopping
507°C,i.e.500°CbiggerthanonMars(Table2).Theupshotisthatneither
planet’ssurfacesupportsliquidwater.InthethinaironMars,apuddlewould
boil(orrapidlyevaporate)andfreezeatthesametime,andwatergenerally
existsonlyasiceorvapour.MeanwhileonthescorchingsurfaceofVenus,liquid
waterisimpossible.
Withoutliquidwater,Venusisconsideredbymostastrobiologiststobelifeless.
Afewspeculatethatacidophilemicrobesmightliveinitscloudsofsulphuric
acidparticles.However,Idoubtit.Apartfromthelackofavailablewater,
atmosphericturbulencewouldpullmicrobesdownintoVenus’sinfernoorupto
fatallydesiccatingheights.
WasVenusinhabitedinthepast?
Venusisstillastrobiologicallyinterestingbecauseitmayoncehavehadoceans
andlife.Itshouldhavebegunwithplentyofwaterbecausetheamountsofother
volatilesaresimilartoEarth’s.(Volatilesaresubstancesthatcanbecomegasesat
prevailingplanetarytemperatures.)Forexample,ifyoutookalloftheEarth’s
carbonaterocksandturnedthemintocarbondioxide(CO
2
),theEarthwould
haveaboutninetyatmospheres’worthofCO
2
,likeVenus.Ifyouextractedall
thenitrogeninmineralsontheEarthandaddedittotheEarth’satmosphere,you
wouldgetalmostthreeatmospheres’worthofnitrogengas,whichagainis
similartothatinVenus’satmosphere.Becauseaccretionofhydratedasteroids
wastheultimatesourceofcarbonandnitrogenonVenusandEarth,it’s
reasonabletoinferthatVenusalsogainedlotsofwaterfromthem,astheEarth
did.
UnfortunatelyVenuswasdoomedbyitsproximitytotheSunbecauseofa
runawaygreenhouseeffect.Whenbakedbyintensesunlight,theevaporationof
watercanmakeanatmospheresomoistthatitbecomescompletelyopaqueto
theinfraredradiationemittedfromtheplanet’ssurface.Atthispoint,there’sa
limittothecoolingoftheplanetbyemissionofinfraredradiationtospace,
whichissetbythepropertiesofwaterandaplanet’sgravity.Recentcalculations
suggestthatthisrunawaylimitisabout282WattspersquaremetreforEarthand
afewWattslessforVenus.
Tounderstandtherunawaylimit,considerturningEarthintoVenus.As
mentionedearlier,thesolarfluxattheEarth’sorbitis1,366Wattspersquare
metrebuttheEarthreflects30percentofthisandsoabsorbsonly70percent.
Thenanadditionalreductionof50percentcomesfromhavingonlyhalfofthe
Earthindaylight,andafurther50percentdecreaseaccountsforglancing
sunbeamsonEarth’scurvedsurface.Puttingallthesefactorstogether,theEarth
absorbsanet(0.7×0.5×0.5)×1,366=240Wattspersquaremetreofsunlight.
Whenstable,theEarthemitsthesameamountofenergyintospaceinthe
infraredandsokeepsaconstantglobalaveragetemperature.Butimagineifthe
EarthweremovedtotheorbitofVenus,wheretheabsorbedsunlightpersquare
metrewoulddoublefrom240to480Watts.Theoceanwouldbecomelikeahot
bathandthesteamyatmospherewouldreachtherunawaylimitwhereonly282
Wattspersquaremetrecanradiateintospace.Withmoreenergypersquare
metrecomingin(480Watts)thangoingout(282Watts),theEarthwouldsimply
gethotterandhotter.Theentireoceanwouldevaporateandsurfacerockswould
melt.Atthatpoint,near-infraredlightfromasearingupperatmospherewould
shinethroughtospace,sothattheincomingsunlightandoutgoingradiation
wouldcomebackintobalanceandthesurfacetemperaturewouldplateauaround
1,200°C.ThisnastyprocessiswhatwethinkhappenedtoVenus.AnyVenusians
wouldhavebeentoast.
TherunawaywouldalsocauseVenus’soceantovanish.Intheupperrunaway
atmosphere,ultravioletlightwouldsplitwatervapourintohydrogenandoxygen.
Hydrogenissolightthatitwouldescapeintospace,draggingalongsome
oxygen,whileanyoxygenleftbehindwouldoxidizehotrocksbelow.Eventually
rockswouldsolidify,butbythistimetheatmospherewouldbeloadedwith
carbondioxideandnitrogenreleasedfromthehotsurface.Theendresultwould
betoday’shellishVenus.
Beforetherunaway,lifemighthavethrivedinVenus’soceans.Recallthatthe
earlySunwaslessluminous.So,afewhundredmillionyearsmighthavepassed
beforetherunawaycommenced.Unfortunately,itmightbetrickytoproveiflife
everexisted.Inthe1990s,radaronNASAsMagellanspacecraftmappedthe
Venusiansurface.Oldersurfaceshavemorecraters,sothedensityofimpact
craterswasusedtoestimatetheageofthesurfaceasafairlyuniform600–800
Ma.Venus’ssurfaceappearstohavebeenrepavedbylavaallatonce.Apossible
reasonisthatwhereasEarth’sinternalheatisreleasedcontinuouslybycreating
newseafloor,onVenusinternalheatmightperiodicallybuildupuntiltheinterior
becomessohotthatlavaeruptseverywhere.Venusmightbehavelikethis
becauseunlikeEarthitdoesn’thavewatertolubricateplatetectonics.
Resurfacingwoulddestroyanyfossils.However,subtlegeochemicaltraces
mightremain.ToinvestigateifVenushadlife,thebestoptionwouldbeto
collectsamplesofVenusianrocksandreturnthemtoEarthinafuturespace
mission.
WateryMars:anabodeforlife?
ManyhaveimaginedMars,unlikeVenus,asapotentialabodeforlife.Before
theSpaceAge,basicparameterswereknown,suchasMars’s24.66hourday,a
yearof1.9Earthyears,andgravitythatis40percentofEarth’s.Notmuch
aboutthesurfacewascertainuntilthefirstsuccessfulspacemission,theflybyof
NASAsMariner4in1965,revealedaheavilycrateredsurfaceliketheMoon.
Thisputadamperonhopesforlife.Then,intheearly1970s,NASAsMariner9
orbiterphotographeddried-uprivervalleysandextinctvolcanoes,suggesting
thatMarshadoncebeenquiteEarth-likeafterall.Butsoonthependulumswung
backintheotherdirection.TheVikingmission,consistingoftwoidentical
orbitersandlanders,reachedMarsin1976,andfailedtofindlife,aswediscuss
later.
Afterahiatusinthe1980s,explorationwasrevived.MarsGlobalSurveyor,
whichorbitedfrom1997to2006,mappedMarsandimagedsedimentarylayers
thatimpliedmanygeologiccyclesoferosionanddeposition.Intheearly21st
century,theMarsOdysseyandMarsExpressorbiters,alongwithMars
ReconnaissanceOrbiter,discoveredareasofclaymineralsandsalts,whichmay
haveformedinliquidwater.TwinMarsExplorationRoverslandedin2004,and
foundfossilizedripplesfrompastliquidwaterandsedimentaryrocks,whilein
2008,NASAsPhoenixLanderdugupsubsurfaceiceinapolarregionand
measuredsolublesaltsinthesoil.Finally,in2012,CuriosityRovertrundled
towardsa5-kilometre-highmountainofsedimentarybedswithina150-
kilometre-diametercraternamedGale.Itfoundmudstonesdepositedfromwater
thatcontaintheSPONCHelements(Chapter1)neededforlife.
Nowadays,astrobiologicalinterestinMarsconcernseitherbiologicaltraces
frombillionsofyearsagoormicrobial-likelifethatmiserablyendures
underground.Pre-SpaceAgehopesofanEarth-likeMarstodayhavebeen
replacedwiththerealityofacold,windswept,globaldesertwithduststorms,
dailydustdevils,andnorainfall.
Mars’scurrentsurfaceishostiletolifeforthreereasons.First,whileiceexists
forsure,noliquidwaterhasbeenunequivocallyidentified.Thepolarcapsare
waterice,toppedwithcarbondioxide‘dryice’thatgrowswhenabout30per
centoftheatmospherefreezesatthewinterpole.Also,abovemidlatitudes,ice-
cementedsoilorpermafrostliesjustbeneaththesurface.Inthetropics,
afternoontemperaturesinthetopcentimetreofsoilriseabovefreezing,but
there,iceturnstovapourbeforemeltingtemperaturesarereached.Asecond
problemisnoozonelayer,allowingharmfulultravioletsunlighttoreachthe
surface.Third,chemicalreactionsintheatmospheremakehydrogenperoxide—
thesamechemicalusedinhairbleach.Hydrogenperoxidemoleculessettleto
thesurface,wheretheycandestroyorganics.
Whilethesurfaceisunpromising,geothermalheatundergroundmightallow
liquidwaterandlifetoexist.Infact,from2004,reportsofatmosphericmethane
averagingtenpartsperbillionbyvolumeledtoexcitementthatsubterranean
methanogensmightbepresent.SunlightreflectedfromMarsgatheredby
telescopesandMarsExpressappearedtoshowabsorptionbyatmospheric
methane.However,themethanesignalwasbarelydistinguishableandsome
scepticalscientists(includingme)doubtedwhethermethanewasreallypresent.
Subsequently,theCuriosityRoverhasfailedtodetectmethanedowntolevelsof
onepartperbillion.
Indiscussingpastlife,astrobiologistsrefertotheMartiangeologicaltimescale,
whichisdividedintoaeonscalledthePre-Noachian(before4.1Ga),Noachian
(4.1toabout3.7Ga),Hesperian(3.7to3.0Ga),andAmazonian(since3.0Ga).
SurfacesonMarsareplacedintoeachaeonaccordingtoimpactcraters.Older
surfaceshaveaccumulatedbiggerandmorenumerouscraters.Infact,thedates
boundingeachaeonactuallycomefromtheMoon.Theagesofrocksbrought
backbytheApolloastronautsareknownfromradioisotopesandthesecorrelate
lunarcrateringdensitieswithtime.Astronomicalcalculationsthataccountfor
moreimpactorsonMarsthantheMoonallowthelunarcorrelationtobe
extendedtoMars.
EvidencethatliquidwaterusedtobepresentsuggeststhatMarswasoncemore
habitablethantoday.Imagesshowfluvial(stream-related)featuresinthe
landscape,includinggullies,dried-uprivervalleys,deltas,andenormous
channels.Also,thesoilandrockscontainmineralsthatforminthepresenceof
liquidwater.
Gulliesareincisionsoftenstohundredsofmetreslengthonthewallsofcraters
andmesasbetween30and70°latitudesinbothhemispheres.Becausegullies
lacksuperimposedcratersandsometimesflowoversanddunes,theymustbe
veryrecent.Initially,theywerethoughttoformwhenicemelted.However,
imagesshowthemformingwhencarbondioxidefrostvapourizes,presumably
releasingadryflowofsoilandrocks.
Farmoreancientfeaturesarevalleynetworks,whicharedried-upriver-like
depressionsthatspreadoutintree-likebrancheswithtributaries(Fig.8).Most
inciseheavilycrateredNoachianterrainandthey’re1–4kmwideand50–300m
indepth.Thedensityoftributariesisfarlessthanformostterrestrialrivers,but
sometimesenoughtoinferthedrainageofrainormeltwaters.Inothercases,
valleyswithstubbytributarieswereprobablyformedbysapping,when
underground(melt)watercausederosionandcollapseoftheoverlyingground.A
fewvalleysendindeltas.
TheNoachianlandscapeconsistsofcraterswithdegradedrimsandshallow
floors.Whatcausedthiserosionisunclear.Valleynetworksareincisedontop
andsoweren’tresponsible.OncetheHesperianstartedat3.7Ga,erosionrates
droppeddramaticallyandvalleynetworksbecamerare.
TowardstheendoftheHesperian(around3Ga),outflowchannelsappeared
(Fig.8).Channelsformfromfluidflowconfinedbetweenbanks,lacking
tributariesandemergingfromasinglesource,unlikerivervalleys.Theoutflow
channelsarehuge:10–400kmwidth,upto1,000kilometresorsolong,andup
toseveralkilometresdeep.Mostbegininchaoticterrainwheretheground
collapsed,sometimesincanyonsorchasmswheretherearemountain-sized
heapsofsulphatesalts,whichmightbeevaporationresiduesfromsaltywater.
8.a)ValleynetworksonMars.Ontheleftistheerodedrimof456-km-
diameterHuygensCrater.Theimageiscentredat14°S,61°E,north
upwards.Scalebar=20km;b)OutflowchannelRaviVallis(0.5°S,318°E)
about205kmlong
Theleadingexplanationforoutflowchannelsisthattheyformedfrom
floodwaterswhenundergroundicemeltedoraquifersburst.However,they
require10–100timesmoreflowthanthebest-knownterrestrialanalogue:flood-
carvedlandacrosseasternWashingtonState,USA,whichearlysettlerscalled
scablands.Thescablandsformedattheendofthelasticeagewhenice
dammingofalargelakeperiodicallyruptured.
It’snoteasytoexplainthewaterneededtoerodeMars’soutflowchannels.
Estimatessuggesttheequivalentofaglobaloceanseveralhundredmetresdeep,
whichisfarmorewaterthanexistsasicetoday.Sincethechannelsflowtothe
northernlowlands,somescientistsspeculatethatanoceanformedthere.In
contrast,aminoritythinksthattheoutflowchannelswerenotcarvedbywater
butbylavas.ChemistrysuggeststhatMartianlavasshouldhavebeenrunny,
turbulent,anderosive.
Apartfromancientvalleynetworksandoutflowchannels(ifwatereroded),
mineralsprovideotherevidenceforawetterearlyMars.Everyoneknowshow
letteringonoldgravestonesdisappearsbecauseofchemicalreactionswithwater.
Suchreactionschangeordissolvemineralsinchemicalweathering.Muchofthe
Martiansurface,likethatonVenusandtheEarth’sseafloor,ismadeofbasalt,a
dark-colouredigneousrockrichinironandmagnesiumsilicateminerals.When
basaltischemicallyweathered,alterationmineralsareproduced,suchasclays.
Sothepresenceofalterationmineralsmeansthatliquidwaterwaspresent,
sometimeswithaspecificpH.Forexample,alkalinewaterstendtoproduceclay
mineralsfrombasalt.
Hydrous(water-containing)alterationmineralshavebeenidentifiedfrom
analysisofinfraredradiationemittedandreflectedbyMars’ssurface.Butonly
about3percentofNoachiansurfaceshavehydrousclaysandcarbonates.
SulphatemineralsdominatelateNoachianorHesperianareas,whilereddish,dry
ironoxidesarecommononyoungerAmazoniansurfaces.Thispatternmight
implythreeenvironmentalepochs.Inthefirstepoch,alkalineorneutralpH
watersweatheredbasaltandmadeclays.Duringthesecond,sulphuricacidwas
derivedfromvolcanicsulphurgasesandmadethesulphates.Thethirdepoch
continuestodaywithacold,dryenvironmentandrust-colouredsurfaces.
OneofthetwinMarsExplorationRovers,namedOpportunity,actuallylanded
nearNoachiansulphates.Millimetre-sizedspheresoftheironoxidehematite
(Fe
2
O
3
)wereembeddedinsulphatelayers,likeblueberriesinamuffin.The
hematiteprecipitatedfrommineralscarriedinwaterpercolatingunderground
about3.7billionyearsago.Also,ankle-deepwaterappearstohavepondedon
thesurface,leavingbehindripplesinthesediment.TheotherRover,named
Spirit,foundevidenceofancienthotspringsontheoppositesideoftheplanet.
TheearlyatmosphereandclimateofMars
EvidenceofliquidwaterleavesuswonderingwhethertheclimateonearlyMars
waswarmandwet.Scientistsdisagreeandfallintotwocamps.Onegroup
arguesthatMarshadawarm,wetclimatefortensorhundredsofmillionsof
years.Theothermaintainsthattransientmeltingoficeinacoldclimatecould
accountforwhatisseen.
Thefirstscenarioisobviouslymorefavourableforlife.Butunfortunately,no
onehasyetexplainedhowearlyMarswaskeptwarmformillionsofyears.
Some3.7billionyearsago,theSunwas25percentfainter.Agreenhouseeffect
ofabout80°CwouldhavebeenneededtokeepearlyMarsjustabovefreezing,
comparedtoEarth’smodern33°Cgreenhouse.It’sgenerallythoughtthatany
atmospherichydrogenshouldhaveescapedintospacerapidlywhenMars
formed,leavingtheatmosphereoxidizedandfullofcarbondioxideand
nitrogen.Marscouldn’teverhavehadanextremelythickVenus-likeatmosphere
becausecarbondioxidewouldcondenseintoiceandformcloudsatMars’s
distancefromtheSun.Athickcarbondioxideatmosphereisalsogoodat
scatteringsunlightbacktospace,whichcoolsthesurface.Socarbondioxide
can’tprovideanearlywarmclimate.Analternativesuggestionisthatvolcanic
sulphurdioxidewasthekeygreenhousegas.However,sulphurdioxidedissolves
inrainandwouldbeflushedfromtheatmosphereifMarsbecamewet.Also,
atmosphericreactionsathighaltitudesmakeafinesuspensionofsulphate
particlesfromsulphurdioxide,whichreflectssunlightandcoolstheplanet.Such
coolinghappenedonEarthduring1991–3asaresultofvolcanicemissionsfrom
MtPinatubointhePhilippines.
Theothercampproposesmanymechanismsthatallowliquidwaterinacold
climate.Theynotethatimpactswouldhavevaporizediceintosteam,which,in
turn,wouldhaveproducedrainfallthaterodedrivervalleys.Also,erosionmight
havebeenproducedbylocalsnowmeltasaresponsetopastfortuitous
combinationsofMars’saxialtiltandorbitalshape.Thesecharacteristicsare
perturbedovertimebythegravitationalinfluenceofotherplanets.Thetiltofa
planet’saxiswithrespecttotheplanet’sorbitalplane,e.g.23.5°forEarth,causes
theseasonsbecauseonehemispheregetsmoresunlightatonepointintheorbit
thantheother.UnlikeEarth,Marshasnobigmoontostabilizeitsaxis(itstwo
tinymoonshavenegligibleeffect),soMars’stilthasvariedbetween0°and80°
overthelast4.5billionyears.Athightilts,summertimepolaricefacestheSun
andvapourizes.Aircurrentstransportthevapourtothecoldtropicswhereit
snows.Sunlightduringotherseasonsoratlowertiltscouldthenproduce
meltwatersandfluvialerosion.Moderatetiltsinthelastfewmillionyearsmight
explainrelictmidlatitudepatchesofdustthatwereonceice-cemented.Finally,
saltywateronMarscanremainliquidfarbelow0°C.OnEarth,wespread
sodiumchlorideonicyroadsinwinterbecauseitmeltsicedownto–21°C.
Anothersalt,perchlorate,whichwasdetectedinMartiansoilbythePhoenix
landerinmagnesiumorcalciumform,candepressthefreezingpointofwater
below–60°C.
WhateverthetruthaboutearlyMars,Mars’ssmallsizeultimatelyspoiledits
habitability.Becausesmallobjectscoolfasterthanlargeones,internalheatwas
lostrapidly,sothatwidespreadvolcanismceasedlongago.Withoutvolcanism,
Marscouldn’trecyclecarbondioxide,leavingthegastobeconvertedinto
carbonates,whicharepresentbutnotintheabundanceexpectedifathickcarbon
dioxideatmospherehadallbeentransformed.So,inaddition,partoftheearly
Martianatmospherewasprobablyblastedawaytospacebylargecometand
asteroidimpactsinso-calledimpacterosion.Theatmospherewasvulnerable
becauseofMars’slowgravity.Alongwithmoregradualescapeofgasestospace
later,Marswasultimatelyleftwithitsthinatmosphere.
LookingforlifeonMars
Westilldon’tknowifMarshaslifeoreverhadlife,despiteattemptstofindit.
TheVikinglanderstriedtodetectlifein1976,and,sincethe1990s,scientists
havelookedforsignsofpastlifeinMartianmeteorites.
EachVikinglanderhadthreeexperimentstodetectmetabolismandafourthto
findorganicmolecules.Thefirst,thecarbonassimilationexperiment,examined
ifMartianmicrobesobtainedcarbonfromair.Martiansoil(sometimesmixed
withwater)wasexposedtocarbondioxide(CO
2
)andcarbonmonoxide(CO)
gasesbroughtfromEarth,withcarbon-14,aradioactiveisotope,ineachgas.
Afterwards,thesoilwasfoundtohaveincorporatedcarbon-14.Whena‘control
experiment’wasdonewherethesoilwasfirststerilizedat160°C,thesoilstill
tookupcarbon-14,whichsuggestedthatinorganicchemistrywasresponsible,
notMartianmicrobes.Asecondtestwasthegasexchangeexperiment,which
monitoredMartiansoilandasolutionoforganicsbroughtfromEarthtoseeif
gasesweregeneratedfrommetabolism.Strangely,O
2
wasreleased,butitwas
alsoreleasedfromsterilizedsoil.Evidently,chemicalsinthesoildecomposed
intoO
2
withwaterorheat.Thethirdinvestigation,thelabelledrelease
experiment,addedorganicscontainingcarbon-14tosoil.IfMartians
metabolizedtheorganics,theywouldgiveoffcarbon-14-containingCO
2
.
Radioactivegaswasemitted,whilesterilizedsoilreleasednoradioactivegas.At
facevalue,thiswasapositivedetectionoflife.Butmostscientiststhinkthatsoil
oxidantsreactedwithorganicstomakeCO
2
andwereinactivatedbyheat.A
reasontopreferthisexplanationcamefromthefourthexperiment,inwhicha
sortofelectronicnose,calledagaschromatographmassspectrometer,
identifiedmoleculeswaftingoffheatedsoil.Itfoundnoorganicmaterialinthe
soiltoadetectionlimitofafewpartsperbillion.
Overall,themoralisthatbeforelookingforextraterrestrials,youneedto
understandtheinorganicchemistryoftheenvironmenttoavoidfalsepositives.
Infact,Harold‘Chuck’Klein(1921–2001),wholedtheVikingbiology
experiments,toldmethathewantedtodothiswhentheVikingmissionwas
conceived,butmanagersatNASAwhocontrolledfinancesinsteadinsistedon
lookingforlifedirectly.
WhiletheVikingresultswerebeingponderedinthe1980s,aremarkable
discoverywasmade:therewerealreadyrocksfromMarsonEarth—Martian
meteorites!ImpactsknockrocksoffMarsandsomeofthesefallontheEarth.In
fact,about50kglandseveryyear,mostlyintheocean.Gassealedwithinsome
meteoritesduringminormeltingassociatedwithimpactejectionprovesa
MartianoriginbecauseitmatchestheatmospheremeasuredbytheViking
landers.OtherMartianmeteoriteswithouttrappedgashaveatripleoxygen
isotope(
16
O,
17
O,
18
O)compositionintheirsilicatemineralsthatisuniquetoall
Martianmeteoritesandidentifiesthemlikeafingerprint.By2013,sixty-seven
Marsmeteoriteswereknownbutthelistkeepsgrowing.
In1996,possiblesignsofpastlifeinALH84001werereported.ThisMartian
meteoritewasthefirstone(the‘001’)collectedintheAllanHills,Antarctica
(the‘ALH’),ina1984expedition(the‘84’).ALH84001crystallizedasan
igneousrockonMarsat4.1Ga.Insidetherockaresomecarbonateglobules
about0.1mmacrossthatformedat4.0–3.9Ga,andwithinthemarefour
possibletracesoflife:allegedmicrofossils;carbonatessaidtobeprecipitatedby
microbes;tracesoforganicscalledpolycyclicaromatichydrocarbons(PAHs),
whicharemadeofhexagonalringsofcarbonatoms;andcrystalsofthemineral
magnetite(Fe
3
O
4
)saidtobesimilartothosewithincertainbacteria.OnEarth,
magnetotacticbacteriamakemagnetitecrystalsinsidetheircellswithshapesthat
evolutionhashonedintomagnets.Thebacteriausemagnetitecompassesto
movealongtheup–downcomponentoftheEarth’smagneticfieldinorderto
findaboundarybetweenaloweroxygen-poorzonebelowandanupper
oxygenatedzone,whichoptimizestheirmetabolism.
Insubsequentyears,researchhascastdoubtonallfourclaims.Thealleged
microfossilsarerod-shapedstructuresthatmerelylooklikemicrobes.Scientists
havesincefoundinorganicmineralsurfaceswithsimilarshapes.Also,the
structuresinALH84001areabouttentimessmallerthanterrestrialmicrobesand
probablybeyondtheminimumvolumeneededforessentialbiochemistry.An
evaporatingfluidcanproducethecarbonatesintheglobulesandsothere’slittle
reasontoinvokebiogenicsalts,proposedasthesecondfeature.Regardingthe
thirdlineofevidence,analysesshowedthatmostPAHsgotintothemeteorite
whileitwassittinginAntarcticafor13,000years.PAHsareubiquitous
atmosphericpollutantsproducedfromburntorganicmaterial.Meteoritesare
darkandabsorbsunlightwell,sotheycansitinpuddlesinAntarcticiceduring
summertime,allowingwaterandchemicalstoinfiltratecracks.Ifthereare
MartianPAHsintheinteriorofALH8401,thesecouldhavebeenmade
inorganically.Therockwasheatedbyimpactsthatoccurredbeforetheejection
impact.Heatingshouldhavereleasedcarbon-bearinggasfromthecarbonates,
whichcanreactwithwatertomakeorganicmatterwithoutlife.Thefourth
argumentconcernedmagnetite.Itturnsoutthatonlyatinyfractionofmagnetites
inALH84001havebiogenic-likeshapes.Somescientistsarguethatmagnetiteof
manyshapes,includingbacteria-likeones,formedduringheatshockspriorto
ejectionwhenironcarbonatedecomposedintomagnetite.
ThecontroversyaboutALH84001showshowdifficultitistoprovethat
microscopiclifeexistsinoldrocks.Butsincemostterrestrialrockslackfossils,
absenceoflifeinALH84001doesn’tmeanthatlifeonMarsdidn’texist.We
needtokeeplooking.
Ceres:aseriouscandidateforhabitability?
BeyondMarsandwithinJupitersorbitisanasteroidbeltofmillionsofsmall
rockybodies,includingthelargest,Ceres,some950kmindiameter,at2.8AU
fromtheSun.Ceres’ssurfacecontainsclaymineralsandwaterice.AsonMars,
clayssuggestpastliquidwater.AleadingmodelforCeres’sinternalstructureis
arockycorethat’ssurroundedbya100-km-thickshellofice.Justabovethe
core,theremaybeasubsurfaceocean.Thiscouldbeverysalty,allowingits
persistenceatsub-zerotemperatures,andperhapsitonceflowedtothesurface.
CouldlifeexistinhydrothermalventsatthebottomofCeres’socean?Ceresis
sosmallthatthere’sprobablylittleinternalheatavailabletoday,soanybiomass
wouldbeextremelymeagre.Butperhapsinthepast,Cereswasmorehabitable.
NASAsDawnmissionwillarriveatCeresin2015.Spectraofinfraredemission
andreflectedsunlightshouldgiveusmoredetailaboutthesurfacecomposition
andhabitability.
TheicyGalileanmoonsofJupiter
BeyondtheasteroidbeltisJupiter,whichisprobablynotarealistictargetfor
astrobiologyforreasonsgiveninChapter5.ButJupiterhassixty-sevenmoons
andthreemightbehabitable.Galileodiscoveredthefourlargestmoonsin1610,
whichareIo,Europa,Ganymede,andCallisto,goingoutwardsfromJupiter
(Fig.9).IoandEuropaaresimilarinsizetotheMoon,whileGanymedeand
CallistohaveproportionscomparabletoMercury.Theouterthreesatelliteshave
rockyinteriorscoveredwithice,whileIo’sexteriorisjustrocky.Io,Europa,and
Ganymedeprobablyalsohaveironcores.
9.TheGalileanmoonsofJupiter:Io,Europa,Ganymede,andCallisto
IoisthemostvolcanicbodyintheSolarSystem,withvolcanoesspewing
sulphurdioxide,whichfreezesontoIo’ssurface.ButIoisdevoidofliquidwater
andsurelylifeless.IoissovolcanicbecausegravitationalforcesfromJupiter
varyasIogoesroundinanellipticalorbit,squeezingIolikeastressball.This
tidalheatingofIoiscausedbyfrictionwhensolidmaterialmovesupanddown,
similartothewaythatwatersloshesinEarth’soceanictides.Io’sorbitisforced
tobeellipticalbecauseofperiodicgravitationalproddingfromEuropaand
Ganymede.ThetimesthatGanymede,Europa,andIotaketomakeorbitshavea
ratioof4:2:1,whichcausesthemoonstolineupcyclicallyandnudgeeachother
gravitationally.ThisrelationshipisthecalledtheLaplaceresonance,after
Pierre-SimonLaplace,theFrenchmathematicianmentionedinChapter2.
Europaalsohasanorbitthat’sforcedtobeellipticalandsoithastidalheating
too.TheheatingissmallerthanthatofIobecauseEuropaisfurtherfromJupiter.
TheaveragetemperatureofEuropa’sicysurfaceis–155°C,buttidalheatcan
maintainliquidwaterdeepundertheice.Impactcrateringdensitiessuggesta
surfaceageofonly20–180Maconsistentwithslushfrombelowrepavingthe
surface.Imagesshowchaoticterrain,i.e.disruptedsurfaceswithblocksthat
haveshifted,whichsuggestthatsubsurfaceicemightconvect.Aglobalnetwork
ofstripesandridgesincludesbandswherethesurfacehasrippedapartandice
appearstohavebeenextruded.Magnesiumsulphatesaltispresentonthe
surface,perhapsoriginatingfromwaterswithin,whileunknownmaterialscause
reddishstreaks.
ThekeyevidenceforEuropa’ssubsurfaceoceancomesfrommagnetic
measurementsbyNASAsGalileospacecraft,whichorbitedJupiterfrom1995
to2003.UnlikeEarthorJupiter,Europa’sinteriordoesn’tcreateitsown,
intrinsicmagneticfield.ButinpassingthroughJupiterslargemagneticfield,
electricalcurrentsareinducedinsideEuropa.Inturn,thesecurrentsgeneratea
weakandvaryinginducedmagneticfield.Thestrengthofthisfieldrequiresthat
anelectricallyconductingfluidexistwithin200kmofEuropa’ssurface.The
mostlikelyexplanationisasaltyoceanuptotwicethevolumeofEarth’socean.
Thelargestcraterssuggestthattheicecoverabovetheoceanisatleast25km
thick.However,someterrainthatpokesupmightbeproducedbyupwellingice
thatmeltsafewkilometresbelowthesurfaceandthenrefreezes.Soshallow
lens-shapedlakesmightexist.
Besidesliquidwater,thepossibilityoflifedependsonenergysources,
interfaces,andtheavailabilityofSPONCHelements.Europaprobablyhasa
goodsupplyofbiogenicelementsifitwasmadeofmaterialsimilartocarbon-
richmeteoritesorcomets.Moreover,heat-releasingreactionsofwaterandarock
seafloorcouldsupplyenergyaswellasnutrients.Radiogenicheat,whichis
producedbythedecayofradioactiveisotopeswithintherock(suchas
potassium,uranium,andthorium),mightproduceseafloorventsthatsupply
carbondioxideandhydrogenforchemoautotrophs.
Theavailabilityofoxygenintheoceanwouldalsopermitbiologicaloxidationof
ironandhydrogenontheseafloor.Therearetwosmallsourcesofoxygen.
ChargedparticlestrappedinJupitersmagneticfieldslamintoEuropa’sicy
surfaceandbreakwatermolecules,releasingoxygen.Iftheicechurns,someof
theoxygencouldbecarrieddowntotheocean.Otheroxygencomesfromwater
moleculessplitbyradiationfromradioactiveelements.EstimatesofEuropa’s
biomassvaryfromaroundathousandtoabilliontimeslessthanonEarth.Some
optimistsevenspeculatethattheremightbeenoughoxygenforanimal-like
creatures!
MeasurementsofinducedmagneticfieldsonGanymedeandCallistoalsoimply
subsurfaceoceansbutdeeper,belowroughly200kmand300kmdepth,
respectively.Ganymede’saveragesurfaceageisabout0.5Ga,soitsoceanhas
probablynotgushedtothesurfacerecently.BecauseGanymedeisJupiters
largestmoon,itsinteriorsupportshigh-pressureformsoficethatwouldlie
beneaththeocean.Lackofarock–waterinterfacemightbelessfavourablefor
lifethanonEuropa.ThelowertidalheatingonGanymedealsoimpliesasmaller
biomass.
InferenceofasubsurfaceoceanonCallistoissurprisingbecauseCallistolacks
tidalheating.Thewarmthmustberadiogenicandtheoceanmightbeloaded
withsaltsthatlowerthefreezingpoint.Callisto’ssurfaceageis4Ga,sothe
prospectoffindingevidenceforoceaniclifetheremightbeslim.Also,withless
energyavailable,anybiomassshouldbeevensmallerthanthatonGanymede.
EnceladusandTitan:icymoonsofSaturn
Saturnanditssixty-twomoonslieatabouttwicethedistanceofJupiterfromthe
Sun.Thelargestmoons,discoveredbeforetheSpaceAge,are(goingoutwards):
Mimas,Enceladus,Tethys,Dione,Rhea,Titan,Hyperion,Iapetus,andPhoebe.
Enceladusisspecialbecauseithasactivegeology.It’sSaturn’ssixthlargest
moon,butits500kmdiametercouldalmostfitwithinGreatBritain.Enceladus
orbitsSaturntwiceforeveryorbitofDione.Asaresult,periodicgravitational
nudgesfromDioneforceEnceladus’sorbittobeelliptical,resultinginvarying
gravitationalforcesfromSaturnthatflexandheatEnceladus.Forreasonsthat
arenotfullyunderstood,theheatingisconcentratedunderEnceladus’ssouth
pole.There,icyparticlesandgassprayoutofparallelfracturesdubbedtiger
stripes,whichareabout130kmlong,2kmwide,andflankedbyridges.Thejets
containtracesofmethane,ammonia,andorganiccompounds,alongwithsalt.In
fact,thejetssupplyparticlestothe‘E-ring’ofSaturnwithinwhichEnceladus
orbits.
Enceladushasarockycore,anicyshell,andanundergroundseabeneaththe
areaofthejets.Thejackpotcombinationoforganicmolecules,energy,and
liquidwaterimpliesthatlifemightexistinsideEnceladus.Suchlifecouldbe
chemoautotrophic,feedingoffhydrogenproducedbywater–rockreactionsor
hydrogenandoxygenfromwaterthat’ssplitbyradioactivity.Iflifedoesexist
there,methaneororganicmatterinthejetscouldbebiological.
Titan,thelargestmoonofSaturn,isalsoofastrobiologicalinterest.It’ssimilarin
sizetoGanymedeandMercuryandistheonlysatelliteintheSolarSystemto
haveathickatmosphere.Infact,theairpressureatTitan’ssurfaceis1.5bar,
whichis50percentlargerthanthatattheEarth’ssurface.Theatmosphereof95
percentnitrogenand5percentmethaneprovidesa10°Cgreenhouseeffect,but
thesunlightatSaturn’sdistanceisonehundredtimeslessintensethanatEarth,
soTitan’ssurfaceisincrediblycoldat–179°C.
Titan’satmospherecontainsasmoggyhazeofhydrocarbons.Athighaltitude,
ultravioletsunlightbreaksupmethanemolecules(CH
4
)andsubsequent
reactionsbuilduphydrocarbonsincludingethane(C
2
H
6
),acetylene(C
2
H
2
),
propane(C
3
H
8
),benzene(C
6
H
6
),andreddish-brownparticlescontaining
polyaromatichydrocarbons.TheparticlesarecalledtholinsfromtheGreek
tholosfor‘muddy.’
Theproductsderivedfrommethanesedimentoutoftheatmosphere.Infact,
about20percentofTitan’ssurfaceiscoveredintropicaldunesthataremadeof
sand-sizedparticles,whichareatleastcoatedwithorganicsifnotmadeofthem.
Atthepoles,over400lakesaremixturesofliquidpropane,ethane,andmethane,
withextraordinarybeachesmadeofparticlesofbenzeneandacetylene.
MethaneonTitanbehaveslikewateronEarthandformsclouds,rain,andrivers.
MostofourknowledgeaboutTitancomesfromtheCassini–Huygensmission,
whicharrivedin2004.HuygenslandedonTitanin2005,whileCassiniisa
Saturnorbiter.NearthelandingsiteofHuygens,channelswereerodedbyliquid
hydrocarbons(Fig.10).Atthelandingsiteitself,cobbles(presumablymadeof
waterorcarbondioxideice)havebeenroundedasifbytransportinriversof
liquidmethane.Methanerainshouldbeveryinfrequent,butwhenitrains,it
pours.
Becausemethaneisdestroyedbysunlight,Titan’satmospherewouldrunoutof
methaneinabout30–100millionyearsifitwerenotresupplied.Howmethaneis
replenishedisamystery.Theleadinghypothesisisthatmethaneleaksoutof
Titan’sinterior,whichsuggestsgeologicalmovementinsideTitan.
Titanflexestoomuchtobeentirelysolid,whichisevidencethatTitanhasa
subsurfaceocean.TheeccentricityofTitan’sorbitensuresthatTitanissqueezed
bySaturn’sgravity.Theoceanexistsbelowanicycrustoflessthan100km
thickness.ButaswithGanymede,theseafloorshouldbedenseiceratherthan
rockbecauseTitan’smassallowssuchhigh-pressureformsofice.
10.a)Anetworkofchannelsthatappeartoflowintoaplainnearthe
Huygenslandingsite;b)ImageofthesurfaceattheHuygenslandingsite.
Stonesintheforegroundare10–15cminsizeandsitonadarker,fine-
grainedsubstrate
TwotypesoflifemightexistonTitan:Earth-likelifeinthesubsurfaceocean;
andweirdlifeinthehydrocarbonlakes.Regardingthefirstpossibility,when
Titanformed,theheatreleasedfromthecaptureofsmallerbodiesshouldhave
createdliquidwateronthesurfacetemporarily.Perhapswater-basedlifeevolved
andsurvivedasanoceanretreatedunderground.
Weirdlifewouldbelimitedinbiochemicalcomplexitybecauseoxygen,whichis
scarce,isrequiredforsugars,aminoacids,andnucleotides.Titancapturessome
oxygen-containingmoleculesfromspace,butthesupplyistiny.Hypothetical
lifeincoldhydrocarbonsolventsmightuseacetyleneforenergy.OnEarth,
acetyleneburnswithoxygeninweldingtorches.ButTitanianscouldmetabolize
acetylenewithhydrogenfromTitan’sair.Thechemicalreaction,
C
2
H
2
(acetylene)+3H
2
(hydrogen)=2CH
4
(methane)
producesenergy,buttheideaisspeculative.Themainproblemforweirdlifeon
Titan,Ithink,isthatliquidhydrocarbonsarepooratdissolvinglargemolecules,
whichareessentialforgenomes.Largermoleculesarelesssolublethansmaller
onesinliquidhydrocarbons.Furthermore,solubilitydeclinesatlower
temperatures.
SomeexoplanetsmighthaveTitan-likeliquidhydrocarbons.Themostcommon
starsarereddwarfs,whicharesmallerandcolderthantheSun.Exoplanetsnear
1AUdistancearoundsuchdwarfswouldhavesurfacetemperaturesintherange
ofliquidhydrocarbons,notliquidwater.Thus,ifweweretodiscoverweirdlife
onTitan,wemightimagineauniverseteemingwithlifeutterlydifferentfrom
ourown.Ironically,wewouldthenbetheweirdlife.
Triton:acapturedKuiperBeltobjectaround
Neptune
AtandbeyondtheorbitofNeptuneat30AU,objectsarecoveredinnitrogenice
(N
2
).Triton,thelargestofthirteenmoonsofNeptune,andPlutoarebothreally
KuiperBeltobjects(KBOs).TheKuiperBeltisaregionoficybodieswithinthe
planeoftheSolarSystemat30–50AUleftoverfromSolarSystemformation.
TherearealsoKBOsscatteredoutto1,000AU.TritonwascapturedbyNeptune
becausetwofeaturesofitsorbitsuggestso:Tritonorbitsintheopposite
directiontoNeptune’srotation,anditsorbitalplaneistippedup157°with
respecttoNeptune’sequator.
Triton’ssurfaceistremendouslycold,around–235°C,becauseitreflects85per
centofsunlight.Anextremelythinnitrogenatmosphereexerts20millionthsofa
barsurfacepressure.This‘airissimplythenitrogenvapourthatwillsitover
nitrogeniceatTriton’sprevailingtemperature.There’salsoalittlemethane
(about0.03percentofthenitrogenconcentration),whichisdestroyedby
ultravioletsunlight,creatingathinsmogofhydrocarbonparticles.Ifmethane
destructionhasbeenoperatingatthesameratefor4.5billionyears,abouta
metre’sdepthoforganicmaterialshouldhaveaccumulated.However,mobile
frostswouldcoverthisup.Areddishtinttosomeoftheicemightbethe
organics.Infact,sunlightsometimesvaporizesnitrogeniceintogeyser-like
plumes,whichcarrydarkparticlesthatareperhapsorganic.Apartfromnitrogen
ice,waterandcarbondioxideicemakeupsomeofthesurface.
Tritonisgeologicallyactivebecausecratercountssuggestthatresurfacingis
only10Maandthereareicystructureslikevents,fissures,andlavas.Wecall
thesecryovolcanic,meaningaformofvolcanisminwhichslushyicecomprises
theequivalentoflavaandmagma.Triton’sdensitysuggestsalargerockycore
thatsuppliessufficientradiogenicheatforcryovolcanismandpotentiallya
subsurfaceammonia-waterocean.
EarlyTritonmayhavebeenmorehabitable.Justaftercapture,Triton’sorbit
wouldhavebeenhighlyelliptical,producinghugetidalheatingfromNeptune
thatshouldhavemeltedanextensivesubsurfaceocean.Tidestendtocircularize
orbitsandtodayTriton’sorbitisclosetocircular.Thus,aftercapture,theice
shellgraduallythickenedastidalheatingdeclined.Today,theoceanmightbe
belowhundredsofkilometresofice,andwouldbeunderlainbyhigh-pressure
formsofice.Butalthoughoceaniclifemightbedeeplyhidden,thepresenceof
cryovolcanismsuggeststhatifwesampleorganicmatteronthesurface,there’sa
chanceoffindingtracesoflife.
DoesPlutohaveanunderworldocean?
Pluto,namedafterthegodoftheunderworld,hasasurfacemainlyofnitrogen
ice,andathinnitrogenatmosphereof8–15microbarsurfacepressure.There’s
alsoalittleatmosphericmethanethatsunlightdestroysjustasonTritonand
Titan.Again,thisproduceshydrocarbonparticles,whichsettleout,possibly
accountingfordarkareasonPluto’ssurface.
AcollisionbetweenPlutoandanotherKBOisbelievedtohavecreatedCharon,
thelargestofPluto’sfivemoons,inaneventanalogoustotheimpactthatformed
theEarth’sMoon.Thus,PlutoisreallyadoubleKBO:CharonishalfPluto’s
sizeandone-ninthitsmass.Every6.4daystheyorbitaroundapointthatlies
betweenthem.AftertheCharon-formingimpact,tidalheatingshouldhave
meltedasubsurfaceoceanonPluto.Plutoprobablyhasarockycorethatmight
supplyenoughradiogenicheattomaintainasubsurfaceoceanofammonia-rich
watertoday.
OrganismsinPluto’soceanwouldbelimitedbyenergy,sothebiomasswouldbe
farsmallerthanonEuropa.Anyoceanisalsoprobablyatdepthsexceeding350
km,anddifficulttoaccessinafuturespacemission,butlifemightbethere.
OurconsiderationofPlutoshowsthatlifemightexistinremarkablyunlikely
places,andmylistinTable1maybetooconservative.Manyofthecoldouter
SolarSystemobjectsusedtobedismissedaspotentialhabitats.Butperceptions
haveshifted.It’spossiblethattherearesubsurfaceammonia-richoceanson
Saturn’smoonRhea,Uranus’slargestmoonsTitaniaandOberon,andperhaps
somelargeKBOs,suchasSedna,whichissimilarinsizetoPlutoyet100AU
fromtheSun.GiventhatsomanyobjectsintheSolarSystemarepotential
refugesforlife,billionsofsimilarpossibilitiessurelyexistthroughoutour
galaxy.
Chapter7
Far-offworlds,distantsuns
Thehuntforexoplanets
BeyondtheSolarSystem,astronomershavediscoveredover3,400exoplanets,
includingcandidatesandconfirmedbodies.It’seasiertodetectlargeones,so
nearlyallarebiggerthanEarthandsomeareratherexotic.HotJupitersare
Jupiter-sizeplanetswithin0.5AUoftheirparentstars,someorbitinginonlya
fewdays.ThenthereareplanetsthatsoundlikeEarthonsteroids,theSuper-
Earths.Theseareuptotentimesthemassofourplanet.Althoughhardertofind,
it’sclearlyjustamatteroftimebeforemanyEarth-sizedexoplanetsbecome
known.Howmanywillbehabitableoreveninhabited?
Ofcourse,beforeidentifyingahabitableexoplanet,youhavetofindexoplanets.
Withthevastdistancesinvolved,thesearchisdifficultbutnonetheless
astronomershavedevelopedtwoclassesofmethods.Thefirst,indirect
detection,looksforstellarproperties,suchaspositionorbrightness,whichare
affectedbythepresenceofunseenplanets.Thesecondisdirectdetectionofa
planetwithanimageoraspectrumofitslight.
Thefourindirectdetectionmethodsare:astrometry,stellarDopplershift,
transits,andgravitationalmicrolensing.Thekeytothefirsttwoisthataplanet
andstarorbitacommoncentreofmass.Forexample,ifaplanetandstarhad
exactlythesamemass,theywouldorbitapointhalfwaybetweenthem.In
reality,aplanetissmallerthanastar,sothecentreofmassisclosertothestar
andperhapsinsideit.Butinallcases,thestarwillfollowalittleorbitaroundthe
centreofmassand‘wobble’evenwhenitsplanetscan’tbeseen.Inourown
SolarSystem,Jupiterstwelve-yearorbitcausestwelve-yearwobblesofthe
Sun’sposition.Saturnaddsinanothersmallertwenty-nine-yearwobble,
correspondingtoitstwenty-nine-yearorbitaroundtheSun.
Astrometrymeasuresthemotionofastarintheskyusingtelescopes.It’s
sensitivetobigplanetsfarawayfromtheirstarsoitcouldbeusedtofind
planetarysystemssimilartoourown.Butyouhavetowaitmanyyearsor
decadestotracktheeffectsofplanetsfarfromtheirstar.
Thesecondtechnique,theDopplershiftorradialvelocitymethod,reliesonthe
factthatwhenthelightofastarissplitintoallitscolours,thespectrumhasdark
bandslikeabarcode.Theelementsinastellaratmosphereabsorbphotons
comingfromthestarsinterior,whichcausethedarklines.Ifthestarmoves
towardorawayfromtheEarth,thelinesshifttohigher(bluer)orlower(redder)
frequencies,respectively.EveryonehasexperiencedtheDopplereffectinsound.
Whenawailingpolicecarapproaches,thesoundwavesarebunchedintoahigh-
frequencysqueal,butafterthecarpassesby,theybecomealow-frequency
drone.Asimilarfrequencyshiftoccurswithlight.Arhythmicredshiftandblue
shiftoflinesinastellarspectrumshowsthatthestariswobblingandhasa
planetaroundit.Thesizeoftheshiftindicatesthemassofaplanet,whilethe
pacinggivesthetimefortheplanettocompleteanorbit.
In1995,DidierQueloz(thenastudent)andhismentor,MichelMayor,fromthe
UniversityofGeneva,detectedthefirstplanetaroundaSun-likestarusing
Dopplershift.ItwasaplanetofatleasthalfaJupitermassorbitingastarinthe
constellationofPegasuseveryfourdays.Itwasatotalsurprise.Nooneexpected
agiantplanetsoclose(0.05AU)toitsparentstarbecausegiantplanets
shouldn’tformthere.Nowweunderstandthatsomeextrasolarplanetsundergo
planetarymigrationearlyintheirhistory(Chapter3)andweendupseeingwhat
survivedthejostling.In2012,DopplerdatasuggestedapossibleEarth-mass
planetjust4.3lightyearsaway,orbitingthestarAlpha-CentauriB.Theplanet,if
present,lies0.04AUfromAlpha-CentauriB,whichissoclosethatitssurfaceis
probablymoltenrock.
OnesubtletywiththeDopplershifttechniqueishowweviewtheplanetary
system.Iftheorbitisface-on(describedasaninclinationofzerodegreesfroma
planeperpendiculartothelineofsight),there’snoto-and-froDopplershift.The
moreedge-ontheorbit,thegreatertheDopplershift.Itbecomesmaximalatan
edge-oninclinationof90degrees.There’softennowayofknowingthe
inclination,soameasuredDopplershiftmightbesmallerthantheidealedge-on
case,allowingusonlytoinferaminimumplanetarymass.TheDoppler
techniqueismostsensitivetobigplanetsclosetotheirstar,sothat’swhymostof
theexoplanetsinitiallydetectedwerehotJupiters.Butwenowknowfromthe
transitmethodthathotJupitersareactuallyonlyatinyminorityofexoplanets,
around0.5to1percent.
Thetransitmethodmeasuresthedecreaseinstarlightwhenaplanetcrossesthe
faceofastar,whichcanhappenifwe’reluckyenoughtoviewanexoplanet
systemvirtuallyedge-on.Suchgeometryisstatisticallyrare,butthereareso
manystarsthatifyougazewidelyandlongenough,transitswillbeseen.From
thedipinstarlight,it’spossibletodetermineaplanet’sdiameterifthestarssize
canbeestimated.Inturn,theplanet’sorbitalperiodanddistancefromitsstar
canbecalculatedfromthecyclingofthedimming.NASAsKeplermissionisa
telescopethatoperatedfrom2009to2013fromanorbitaroundtheSunstaring
continuouslyat145,000mainsequencestarsintheconstellationofCygnus(the
Swan)tolookfortransitswheremostofthestarsare500–3,000lightyears
away.Keplerhasfoundover3,200exoplanetcandidates,whichareconfirmed
bycheckingforperiodicdimmingorusinganotherdetectiontechnique,suchas
Dopplershift.In2017,theTransitingExoplanetSurveySatellite(TESS)
telescopewillbeginexaminingtwomillionstarsfortransits.
Thefourthindirectmethodismicrolensing.Whenanobjectpassesbetweena
distantstarandus,Einstein’sTheoryofGeneralRelativitypredictsthebending
oflightbytheobject’sgravitationalfield.Theforegroundobjecteffectivelyacts
asalens,focusingthelightandmakingthedistantstarappeargraduallybrighter.
Butifaplanetorbitsalensstar,thebackgroundstarcanbrightenmorethan
once.ThissensitivetechniquecanfindEarth-massplanetsatorbitaldistancesof
1.5–4AUaroundastar.Unfortunately,oncethealignmenthashappened,it’s
virtuallyimpossibletofollowupwithmoredetailedmeasurementsbecausethe
objectsaretypicallyverydistant.However,microlensingcangatherstatistics.
Largeplanetsdriftingaloneininterstellarspacecanalsoactaslensobjects,and
oneresultisthatthereappearstobemanyunboundJupiter-massplanetsfloating
betweenthestars.Theselonelyworldswerepresumablyejectedfromtheir
extrasolarsystems.
Forastrobiology,themostimportanttechniquesaredirectdetectionsthatcapture
lightfromaplanet.Directdetectionischallengingbecauseaplanetisadim
bodyclosetoavastlybrighterstar.Nonetheless,telescopesinspace,suchasthe
HubbleSpaceTelescope,andextremelylargetelescopesonthegroundhave
accomplisheddirectdetectionusingacoronograph,whichisamasktoblockthe
starlight.Theterm‘coronograph’comesfromtechniquesthatwereoriginally
usedtoblockoutsunlighttostudytheSun’scorona,whichisthewispyhalo
seeninasolareclipse.Ground-basedtelescopesalsouseadaptiveoptics,which
areprocedurestosenseandcorrectthedistortionscausedbytheshimmeringof
theEarth’satmosphere.
Aspace-basedtelescopehastheadvantageofavoidingtheblursfromtheEarth’s
atmosphere,butthere’sthequestionofwhichpartofthespectrumtoexamine.In
general,planetsemitmostlyinfraredradiationandreflectvisiblestarlight.Ifwe
lookedatourSolarSystemfromafar,theSun,beinghotandbig,wouldoutshine
theEarthbyafactorofabouttenmillionintheinfraredandtenbillioninvisible
light.SolookingforadistantEarthintheinfraredgivesathousandtimesbetter
contrastthaninvisiblelight.Unfortunately,lightalsospreadsandblurs
(diffracts)whenitencountersanyobjectsuchasatelescopeandthisisworse
withinfraredlightthanwithvisible.Fortunately,atechniquecalled
interferometrycancancelunwantedlight.Noise-cancellingheadphonesusethe
sameprincipletosilenceundesirablesoundwaves.
Lightwaveshavecrestsandtroughslikewaterwaves.Nullinginterferometry
usesmorethanonetelescopemirrortopreciselyalignlightwavesarrivingfrom
apointintheskysothatwavecrestscanceltroughsandlightisturnedinto
darkness.Inthisway,starlightcanbe‘nulled’toseeplanets.BothNASAandthe
EuropeanSpaceAgencyhavestudiedspacetelescopes,calledTerrestrialPlanet
FinderandDarwin,respectively,whichuseinterferometrytocaptureimages
andspectraofexoplanetsaroundnearbySun-likestars.
Theresultsofexoplanetsurveystodategenerallyshowthatthenumberof
exoplanetsincreasesatlowermasses,sorocky,Earth-sizedplanetsshouldbe
common.Densityestimatesforsomeexoplanetsalsoallowassessmentof
composition.DensityhasbeeninferredusingmassfromtheDopplermethodand
sizefromtransitmethods.Also,ifatransitingEarth-sizedplanethasatleastone
Neptune-likesibling,amethodcalledtransittimingvariationscanprovidethe
massofplanetsandthusdensity.Evenifthelargerplanetdoesn’ttransitits
parentstarbecauseofitsorbitalinclination,itcancauseslightvariationsinthe
smallerplanet’stransittimesthatallowplanetarymassestobecalculated.With
densitymeasurements,thereareincreasingcluesaboutwhichexoplanetsare
madeofgas,rock,water,orsomemixture.
Thehabitablezone
WhenwewereconsideringlifeintheSolarSystem,weconcentratedonbodies
withliquidwaterbecausewe’resurethat’sarequirementforEarth-likelife,and
thesameideaappliestoexoplanets.Whereasdryplanetswithsubsurfaceoceans
areworthinvestigatingwithspaceprobeswithinourSolarSystem,they
generallywouldn’tbetargetsforexoplanetastrobiology.There’snoprospectin
thenearfutureofsendingspacecrafttovisitexoplanets,soeverythingmustbe
donewithtelescopeslookingatlightfromafar.Consequently,wecaremost
aboutexoplanetswithliquidwaterrightatthesurface.Thesehavethechanceof
abiospherethatpumpslotsofgasesintoanatmosphere.Inprinciple,such
biogenicgasesaredetectableinthelightfromtheplanetandmightindicatelife.
In1853,WilliamWhewellnotedthatEarth’sdistancefromtheSunallowed
liquidwaterbetweenwhathecalleda‘centraltorridzone’andan‘externalfrigid
zone’.Inthe1950s,theAmericanastronomerHarlowShapley(1885–1972)
(whodiscoveredthedimensionsofourgalaxy)alsotalkedaboutazonearound
starswhereplanetscouldhaveliquidwaterontheirsurface.Ifaplanetistoofar
awayfromitshoststariticesover,andifit’stoocloseit’stoohotforliquid
water.Nowadays,thetermhabitablezone(HZ)referstotheregionaroundastar
inwhichanEarth-likeplanetcouldmaintainliquidwateronitssurfaceatsome
instantintime.Wespecifyaparticulartimebecausestarsageandbrightenor
dim,sotheHZmoves.IncontrasttotheHZ,thecontinuouslyhabitablezone
(CHZ)istheregionaroundastarinwhichaplanetcouldremainhabitablefor
somespecifiedperiod,usuallythestarsmainsequencelifetime.
ThewidthoftheHZarounddifferenttypesofmainsequencestars,includingthe
Sun,hasbeenestimated.Theinneredgeissetbyaplanet’ssusceptibilitytothe
runawaygreenhouseeffect,whiletheouterboundaryisusuallydeterminedby
thefailureofgreenhousewarmingwhencarbondioxideiscoldenoughto
condenseintocloudsofdryiceordenseenoughtoscatterawaysunlight.The
latterwasapotentialproblemforearlyMars(Chapter6).FortheSun,theHZ’s
inneredgeisaround0.85–0.97AU,whiletheperipheryis1.4–1.7AU.The
spreadreflectstheuncertaineffectsofwaterandcarbondioxidecloudsatthe
innerandouterborders,respectively.Forexample,cloudsmightcoolaclose-in
planetbyreflectingsunlight,sotheinneredgemightbe0.85insteadof0.97AU.
Forthemostoptimisticouterboundary,MarsresideswithintheHZ.However,
Mars’ssmallsizehasledtoathinatmospherethatcan’tsustainliquidwater.In
otherwords,ifMarshadbeenasbigastheEarth,perhapsitmightbehabitable
today.SoplanetsizemattersandtheHZisnotthewholestoryforhabitability
butjustafirstguide.
Aroundotherstars,theHZiscloserinorfurtheroutdependingonwhetherastar
iscoolerorhotterthantheSun.Forexample,aKstar,whichiscooler(Fig.1),
wouldhaveatighthabitablezonefromabouttheorbitofMercurytothatofthe
Earth.AnevencoolerM-typereddwarfwouldhavethecentreofitshabitable
zoneatonly0.1AUorso,wellwithinthe0.4AUorbitofMercury.
Infact,theHZoffaintMdwarfsissosnugthatplanetswillexperiencetidal
locking,whichoccurswhengravitysetstherotationperiod.Thesame
phenomenonmakesonefaceoftheMoonpointtowardtheEarth.TheMoonis
tidallylockedandspinsonitsaxisonceforeveryorbitaroundtheEarth.The
matchbetweenorbitalandrotationperiods,calledsynchronousrotation,isno
coincidence.TheMoonelongatesslightlyintheEarth–Moondirection,andif
thisorientationdeviated,theEarth’sgravitywouldtwisttheMoon’sskewed
bulgesbackintoalignment.
AsynchronouslyrotatingplanetintheHZofanMdwarfwouldhaveonesidein
sunlightandtheotherinperpetualdarkness.However,thisdoesn’tmakesuch
planetsuninhabitable.Althoughthenightsidewouldbecold,amoderatelythick
atmospherecantransportenoughheattowarmthenightsidewhilecloudcover
cancoolthedayside.Also,alargemoonorbitingsuchaplanetwouldhave
variablesunlight.SotidallockinginthehabitablezonesofMdwarfsisnota
showstopperforlife.That’sencouragingbecauseMdwarfsarethemost
commontypeofstars,makingupaboutthree-quartersofthetotal.
Inrecentyears,assumptionsdefiningtheHZhavebeenquestioned.
Conventionally,theouteredgeisdeterminedbycarbondioxidecondensation.
Butsomelarge,rockyexoplanetsmighthavethickhydrogen-richatmospheres.
WeknowfromstudyingTitanandthegiantplanetsthathydrogenandmethane
behaveasgreenhousegases,andthesecondenseatfarcoldertemperaturesthan
carbondioxide.Also,ontheinneredge,awater-poorplanetwithpolarlakes
ratherthanoceansmightnotsuccumbtoarunawaygreenhousebecauseit
wouldn’thaveenoughwatertogenerateanatmospherecompletelyopaqueto
infraredradiation.Thus,theHZmightbewiderthanwethink.
Isthereagalactichabitablezone?
Someastronomersalsoarguethatthere’sanoptimalregioninthegalaxyfor
habitableplanetarysystemscalledthegalactichabitablezone(GHZ).Theynote
thattheSunistwo-thirdsofthewayfromthecentreoftheMilkyWay,whereas
planetarysystemsnearthedenselypopulatedcentrewouldbeperturbedby
supernovaeorpassingstars.Attheotherextreme,starsnearthegalaxy’sedge
areverypoorinelementsotherthanhydrogenorhelium,andthismightcurtail
planetformation.Astronomershaveanoddconvention(whichsickens
chemists!)ofcallingallelementsotherthanhydrogenorhelium‘metals’;the
‘metal’contentofastariscalleditsmetallicity.Starswithgiantexoplanetstend
tobemetalrich,andsincegiantexoplanetsweretheonesdiscoveredfirst,itwas
initiallythoughtthathighmetallicitywasneededforplanetstoforminanebula.
Morerecently,nocorrelationwithmetallicityhasbeenfoundinstarsofSun-like
massthathaveSuper-Earths.
AnotherproblemwiththeGHZisthatstarsdon’tstayput.Recentresearch
showsthatstarswanderacrossthegalacticdiscasaresultofgravitational
scatteringbyspiralarms.Inanycase,theexoplanetsthatwillbescrutinizedfor
signsoflifeintheforeseeablefuturewillallbeclose,withinahundredlight
yearsofus.SowhateverthevalidityoftheGHZ,it’snotapractical
considerationforfindinghabitableplanetsanytimesoon.
Biosignatures,orhowwefindinhabitedplanets
Ofcourse,thecentralquestioniswhetherwecanfindlife.In1990,NASAs
Voyager1spacecraftwasheadingoutoftheSolarSystemandlookedbackatthe
Earthfrom6.1billionkmaway.Thefamouspictureittookisknownasthe‘Pale
BlueDot’,inwhichEarthissosmallthatit’sasinglebluishpixel.Ifweknew
nothingelse,whatcouldwededuce?Ifwesaidthattheplanet’scolourwasa
mixtureofblueoceansandwhitecloudsthatwouldjustbeguesswork.Howcan
wedobetter?
Generally,todetermineanexoplanet’spropertiesandlookforlife,weneeda
planet’svisibleorinfraredlightspectrum.Differentmoleculesandatomsabsorb
differentfrequenciesinspectra.Thus,wecanlookforatmosphericgasessuchas
oxygenormethanethatcanbeproducedbylife.Suchplanetaryfeaturesthat
indicatelifearebiosignatures.Infact,inthesameyearasVoyager1’s‘PaleBlue
Dot’,theGalileospacecraft,whichwasheadedforJupiter,obtainedspectraof
Earth.Itwasasortofdryrunforexoplanetsandshowedthatyoucoulddetect
Earth’satmosphericoxygen,methane,andabundantwater.Thesimultaneous
presenceofoxygenandmethaneisevidenceforlifebecausethesetwogases
shouldquicklyreactwitheachotherunlessabiosphereisproducinggreat
amountsofthem,whichpreventsequilibrium.Thus,wesaythatEarth’s
atmosphereisinchemicaldisequilibrium.That’sthekindofbiosignaturewe
wouldlikefromexoplanets.
Already,someinformationhasbeencollectedaboutexoplanetatmospheresby
lookingatdifferencesinspectrawhentransitingexoplanetspassbehindorin
frontofastar.Thespectrumintheprimarytransitwhenaplanetcrossestheface
ofastarincludeslightpassingthrougharingofatmospherearoundtheplanet.
Sosubtractingthespectrumofjustthestaraftertheplanethaspassedbycan
isolatethespectrumoftheplanet’satmosphere.Alternatively,youtakea
spectrumwhentheplanetisbehindthestarinitssecondarytransitandyou
subtractthatfromaspectrumoftheplanetplusstarwhentheplanetisbesidethe
star.Thisproducesthespectrumofthewholeplanet.Infact,thistechniquehas
beenusedtoobtaininfraredspectraofhotJupiterswiththeSpitzerSpace
Telescope,whichisatelescopeorbitingtheSun.
Withmoresophisticatedtelescopes,wewouldliketodetermineanEarth-like
exoplanet’ssurfacetemperatureandwhetherithasliquidwater.Aspectrum
mightprovidethat,butyoumightonlybeabletoseelightfromcloudtopsorthe
upperatmosphere.Infact,Venusisveiledinthisway.Forexoplanetsthatare
similarlyobscured,we’llneedtoinfertheamountofgreenhousegasesfrom
absorptionlinesinspectraandcalculateasurfacetemperature.Togetherwithan
exoplanet’satmosphericcomposition,youcouldtheninferwhetherliquidwater
ispresent.Itmightalsobepossibletolookforanoceanthroughglint,whichis
thebrightreflectionspotonasmoothbodyofwateratglancingangles.Finally,
weshouldkeepanopenmindaboutanti-biosignatures.Microbiallifewill
readilyeathydrogenorcarbonmonoxidegas,soanabundanceofeitherofthese
mightbeconsideredananti-biosignatureforaplanetinthehabitablezone.
Basically,anti-biosignaturessay‘noonehome’.
Thesearchforextraterrestrialintelligence(SETI)
Adifferentapproachtofindinglifeonexoplanetsistohypothesizethat
technologicalcivilizationsexist.Iftheydo,wecanlookfortheir
communications.Thisendeavouristhesearchforextraterrestrialintelligence
(SETI)ormoregenerallythesearchfortechnosignatures.
In1959,GuiseppeCocconiandPhilipMorrisonadvocatedlookingfor
broadcastsofextraterrestrialcivilizationsusinglargeradiotelescopes.Others
havesincesuggestedwatchingfortransmissionsinvisiblelight.Thereare
practicalreasonstolookonlyforthesesignals.Forexample,X-raysare
absorbedintheupperatmosphere,whereasradioandvisiblelightareeasyto
generateanddetectableacrossspace.ThemainquestionforSETIiswhetherit’s
worthlooking.Arethereenoughcivilizationsoutthere?
In1961,FrankDrake(thenatCornellUniversity)describedawaytoevaluate
thepotentialnumberoftransmittingcivilizationsinourgalaxy.Ifweestimate
theaveragenumberofcivilizationsthatcome‘onaireachyearandtheir
averagelifetime,wecanmultiplythosetwoquantitiestogethertogivethe
currentnumberoftransmittingcivilizations.Togiveananalogy,ifthenumber
ofnewfreshmenstartingatauniversityisaround6,000peryearandtheyspend
anaverageoffouryearsoncampus,thetotalundergraduatepopulationatany
onetimeis6,000×4=24,000.Drakewentfurtherbydevisingsixfactorsthat
determinethenumberof‘freshman’transmittingcivilizationsappearingper
year.Infull,wecalculateN,thenumberofcommunicatingcivilizations,by
multiplyingthesixnumbersandtheaveragelifetime,asfollows:
Ncivilizations=
ThisisthefamousDrakeEquation.ThefirstnumberR,isthebirthrateofstars
suitableforhostinglife.Astronomersobserveabouttennewstarsperyearof
typesG,K,andM.Alltheotherfactorsareasfollows:
f
planet
isthefractionofsuchstarshavingplanets;
n
habitable
istheaveragenumberofplanetsperplanetarysystemthatarehabitable;
f
life
isthefractionofthoseplanetsonwhichlifeoriginatedandevolved;
f
intelligence
isthefractionofinhabitedworldsthatdevelopedintelligentlife;
f
civilizations
isthefractionofthoseworldsthatdevelopedcivilizationscapableofinterstellar
communication;
andListhelifetimeofthosecommunicatingcivilizations.
Severalfactorsintheequationareunknown.But,whattheheck?Let’susethe
DrakeEquationanyway.Currentexoplanetsearchessuggestthatatleasttwo-
thirdsofallstarshaveplanets,solet’stakef
Planet
as2/3.DatafromtheKepler
missionarestillbeinganalysed,butsuggestatleastthat1in100ofplanetsare
habitable,solet’ssetn
habitable
=1/100.Wewillsimplyhavetoguessallother
parameters.LifedevelopedrapidlyonEarth,solet’spresumethatlifeoriginates
onhalfofthehabitableplanets,i.e.f
life
=1/2.Let’ssupposethatthefractionof
biospheresthatdevelopintelligenceisf
intelligence
=1/8.Let’salsoguessthatone
intenintelligentbiospheresdevelopcivilizationscapableofinterstellar
transmissions,sof
civilizations
is1/10.Finally,thelifetime,L,ofcommunicating
civilizationsissociologicalspeculation,butlet’ssay10,000years.Sohow
manycommunicatingcivilizationsintheMilkyWaydoweget?Theansweris
four,from10×(2/3)×(1/100)×(1/2)×(1/8)×(1/10)×10,000.Fine.Butcan
weconstraintheprobabilitiesbetter?
Exoplanetstudieswilleventuallynaildownthesecondandthirdtermsinthe
DrakeEquationandmighthavesomethingtosayabouttheprobabilityoflife
arising,f
life
,ifbiosignaturesaredetected.Optimiststhinkthataplanetwith
liquidwaterandtherightmaterialsdevelopslifeeasily.Atpresent,wedon’t
reallyknow.
Thenexttermconcernsintelligence,sohowprobableisthat?Arelevantissue
mightbethatonlysomebiologicalsolutionsworktosolvespecificproblems.In
zoology,weoftenfindthesametraitsharedbyorganismsinunrelatedlineages,
aresultofwhatiscalledconvergentevolution.Convergenceoccursforspecific
functionsandecologicalniches.Forseeing,eyeshaveevolvedinatleastforty
differentanimalgroups.Adog’slife,ofallthings,isanexampleofaconvergent
niche.TheTasmanianwolf(amarsupial)evolveddog-liketraitssimilartothe
Mexicanwolf(aplacentalmammal).Evolutionaryconvergenceissocommon
thatitmeansthatorganismswithlegs,pairsofeyes,andcertainecologiesmight
beinevitablebecauseonlythesearephysicalsolutionstotheproblemsof
walking,stereovision,andoccupyingparticularniches.
Whethertechnologicalintelligenceisuniqueorconvergentiscontentious.On
Earth,itwasslowtoappear.IttookfourbillionyearstobuildupsufficientO
2
to
allowanimallife.Thenanotherfewhundredmillionyearspassedbefore
technologicallife.Dinosaursreignedforabout170millionyearsandyetwefind
nosignoftechnology,neitherfossiltoolsnordinosaurmicrowaveovens.Onthe
otherhand,therearesomelineageswherebrainsappeartohavehadvalueand
grown.Brainmassitselfisapoormeasureofintelligencebecauseabigbody
oftenneedsabigbraintorunit.Instead,theEncephalizationQuotient(EQ)is
used,whichistheratioofthebrainmassofananimaltothebrainmassofan
averageanimalofthesamebodymass.Thus,anaverageanimalhasanEQof1.
Scoreshigherthan1arebrainyandscoresbelow1morebrainless(Fig.11).
HumansandchimpshaveEQsofabout7.4and2.5,respectively,whichmeans
thattheyhavelargerbrainsthanexpectedbythesefactors.ArabbithasanEQof
0.4,whichwouldpleaseElmerFudd.AsAristotlenoted,‘manhasthelargest
braininproportiontohissize’.
11.Brainandbodymassforsomedifferentmammals.Animalsthatplotto
theleftofthediagonallinehavemorebrainmassthantypical
PalaeontologistshavediscoveredanincreaseinEQincertainlineagesovertime,
mostnotablythegenusHomo.TheEQofHomohabilis,ahomininthatlived
about2Ma,wasonly4,forexample.Intoothedwhales(dolphins,sperm
whales,andorcas),EQincreasedaround35Ma,whentheydeveloped
echolocationtofindfishandfriends.‘Friends’maybethekey.Generally,
intelligenceislargerinsocialanimals,suchas5.3foradolphin.Intelligence
probablyaidssurvivalandgettingamate,which,inturn,promotesmore
offspring.However,there’smuchdebateaboutwhichevolutionarypressuresare
behindintelligence.
AfinalquestionaboutSETIiscalledtheFermiParadox,andwasconceivedby
EnricoFermi(1901–54),theNobelPrize-winningphysicistwhobuiltthe
world’sfirstnuclearreactorin1942.Hisideaarosefromalunchtime
conversation.StarsintheMilkyWaydiscdatefromninebillionyearsago,
whereastheEarthisonly4.5billionyearsold.Thus,ifintelligentlifeis
common,manytechnologicalcivilizationsshouldhavearisenlongbeforeus.
Assumingthattheydevelopedspacetravel(orself-replicatingrobotstodothe
spacetravelforthem),theyshouldhavespreadthroughoutthegalaxybynow.
‘Wherearethey?’Fermiasked.
Fermi’sParadoxhasthreesolutions.Oneisthatwe’realoneintheMilkyWay
becauseonefactorintheDrakeEquationisvanishinglysmall.Asecondisthat
Fermi’spremiseisincorrect.Forallmannerofreasons,civilizationsmightnot
goaroundcolonizingthegalaxy.Finally,therearecivilizationsbutthey’re
hidingtheirexistencefromus(ormostofus).Scientistsparticularlydislikethe
thirdoptionbecauseit’sanadhochypothesisthat’suntestable,butit’sbeloved
ofsciencefictionwriters,tabloidnewspapers,and(accordingtopolls)one-third
ofAmericanadultswhobelievethatextraterrestrialshavevisited.The
reasonableoptionsarethefirsttwo.Obviously,alleffortstofindlifeelsewhere
bearuponthequestionofwhetherwe’realoneandthefirstsolution.IfSETI
succeeds,wemightalsogaininsightintoothercivilizations,ifwecould
understandtheirsignals.Atthemoment,theonlyknownlifeishere,so,like
Fermi,wecanonlywonder,‘Wherearethey?’
Chapter8
Controversiesandprospects
TheRareEarthHypothesis
Thebig,unansweredquestionsofastrobiologygeneratecontroversy.Onedebate
surfacedin2000whenPeterWardandDonBrownlee,mycolleaguesatthe
UniversityofWashingtoninSeattle,publishedabestsellingbook,RareEarth.In
essence,theirRareEarthHypothesisisthatthefortuitouscircumstancesthat
haveallowedcomplexlifeontheEartharesouncommonthatEarthmight
harbourtheonlyintelligentlifeintheMilkyWay.Amongsttheirarguments
werethegoodfortuneofEarthbeingintherightplaceinthegalaxy,having
JupiterinourSolarSystemtocapturecometsthatmightotherwisecollidewith
theEarth,Earth’sunusualrecyclingofvolatilesbyplatetectonicstokeepthe
atmospheregoing,thecontingenciesinobtaininganoxygen-richatmosphere,
andtheluckofhavingalargeMoonthatstabilizestheEarth’saxialtiltandsoits
climate.
RareEarthwasapolemicthatrailedagainsttheCopernicanPrinciple.The
latteridea(namedafterNicolasCopernicus,whoseSun-centredsystemknocked
theEarthfromitsperceivedplaceasthecentreoftheuniverse)holdsthatthere’s
nothingspecialaboutourlocation.Astronomersnoteseveralfactorsinits
favour.First,theEarthissurelyoneofmanyrockyplanetsintheuniverse.
Furthermore,ourSun,aG-typestar,isnotspecialbecausearoundoneinten
starsareGtype.Wealsoliveinahumdrumlocationinthegalaxy,alongoneof
manyspiralarms.Finally,ourgalaxyisunremarkableamongmanyinthe
observableuniverse.AsStephenHawkinghasputit,‘Thehumanraceisjusta
chemicalscumonamoderate-sizedplanet,orbitingaroundaveryaveragestarin
theoutersuburbofoneamongahundredbilliongalaxies.’
AdvocatesoftheCopernicanPrinciplealsonotehowthegreatadvancesof
sciencehighlightthefollyofassumingthatwe’respecial.Weneedthinkonlyof
Galileo’sTheStarryMessenger(1610),whichreportedhisobservationsof
moonsofJupiterandthesurfaceoftheMoon,showingthatthesebodieswere
notheavenlyperfectionsassupposedbytheologianphilosophersbutexplained
bythesamephysicsthatwehaveonEarth.Similarly,Darwin’sOriginofSpecies
(1859)overturnedtheconceitthathumansexistoutsidetherestofbiology.
TheproblemwiththeRareEarthHypothesisisthatitassumestoomuch
knowledgeabouthabitability,whereas,inreality,muchisuncertain.Recently,
forexample,it’sbeendiscoveredthatthewanderingofstarsmeansthatthe
galactichabitablezoneisalessconcreteconceptthanpreviouslythought.The
resultsofNASAsKeplermissionalsoshowthatplanetsaroundotherstarsare
common,includingplentyofJupiter-sizebodies.Theargumentthathavinga
Jupiter-sizedplanetinjusttherightplacelowerstherateatwhichcometsor
asteroidshittheEarthhasalsobeenchallenged.Certainly,Jupitermopsupsome
impactorslikeacosmicvacuumcleaner.In1994,thecometShoemaker–Levy9
smashedintoJupiter,whilein2009and2010,thescarsoffurtherimpactswere
seenonJupiter.It’swellestablishedthatJupiterhelpstodeflectandejectcomets
thatcomefromahaloofsuchicyobjects,theOortCloud,whichsurroundsthe
SolarSystematabout50,000AUdistance.However,near-Earthasteroidsand
cometsfromwithintheplaneoftheSolarSystemrepresentmorethan75per
centofEarth’simpactthreatandJupitercanactuallydestabilizethoseobjects.
SoJupiteristwo-faced.SomecalculationssuggestthatJupiterisactuallyanet
foeratherthanfriend.
OtherargumentsforRareEartharealsoambiguous.IntheSolarSystem,Earth
uniquelyhasplatetectonics.However,forplatetectonics,aplanetmustbebig
enoughtohaveadequateinternalheattodrivetheplatemotionanditprobably
needsseawatertocooloceanicplatesandlubricatetheirmovement.IntheSolar
System,onlytheEarthqualifies.Butthatdoesn’tmeanthatEarth-like
exoplanetsinhabitablezonesmightnotalsobesuitableforsimilartectonics.
WhiletheRareEarthHypothesisiscorrectthatplanetswithoutalargemoon
willsufferlargeraxialtiltvariationsthanEarth,climaticvariationsatlow
latitudesmightbebenign.Infact,thickeratmospheres,moreextensiveoceans,
andlowerrotationratesofanexoplanetcansmooththeclimaticdifferences
betweenpoleandtropicscausedbyavaryingtilt.Finally,thequestionofoxygen
notaccumulatingonotherEarth-likeplanetsmightgointheotherdirectionto
thatassumedintheRareEarthHypothesis.Someplanetsmightbemore
favourableforoxygen-richatmospheresthanEarthbecausetheirvolcanoes
pumpoutasmallerproportionofgasesthatreactwithoxygen.Oxygenmight
buildupmoreeasily.
WhatdoesseemtobecorrectabouttheRareEarthHypothesisisthatmicrobial-
likelifeshouldbemuchmorecommonthanintelligentlife.Microbeshavea
remarkablerangeofmetabolismsandcanliveinafarwidervarietyof
environmentsthancomplexorganisms.Butanydefinitivestatementsaboutthe
prevalenceofcomplexlife—onewayortheother—simplylackdatatosupport
themandweshouldbesceptical.AsCarlSaganfamouslyremarked,‘Itpaysto
keepanopenmind,butnotsoopenthatyourbrainsfallout.’
Prospectsforastrobiologyandfindinglifeelsewhere
Theexcitementofastrobiologyisthatittriestoanswerquestionssuchasthe
originoflifeandwhetherwe’realoneintheuniverse.Withadvancesin
technology,it’sincreasinglylikelythatmajordiscoverieswillbemadeinthe
comingdecades.
Intheareaofunderstandingearlylife,it’slikelythatrealisticself-replicating
genomeswillbemadeinthelab.Thiswouldprovidegreatinsightsintothe
originoflife.SeveralgroupsarestudyingtheRNAWorldoritsvariants.There
arealsoprojectstodrilldeeplyintooldsedimentaryrocksinSouthAfricaand
Australia,whichwillsurelymakenewdiscoveriesabouttheearliestlifeand
environment.
IntheSolarSystem,Enceladusoughttobeoneofthehighestprioritiesforthe
world’sspaceagencies.Enceladushasasourceofenergy(tidalheating),organic
material,andliquidwater.That’satextbook-likelistofthosepropertiesneeded
forlife.Moreover,naturehasprovidedastrobiologistswiththeultimatefree
lunch:jetsthatspurtEnceladus’sorganicmaterialintospace.Technology
certainlyexiststobuildaspacecrafttoswingbyEnceladusandsamplethe
organicsinthejets.Betterstill,thematerialcouldbereturnedtoEarthfor
analysis.
Infact,spacecrafttocollectextraterrestrialsamplesandreturnthemtoEarth,
whicharesamplereturnmissions,arethefutureinunderstandingthehistoryof
MarsandVenusandwhethereitheroftheseplanetswasonceinhabited.A
samplereturnmissionforVenusisinthedistantfuture,butoneforMarsisa
strategicgoalofNASAsandESAscurrentprogrammes.
AroundJupiter,Europaisprobablythebestprospectforlife.Thefirststep
wouldbeaEuropaorbitertostudythemoonindetailanddeterminethe
thicknessoftheiceaboveasubsurfaceoceanorlakelenses.Thenextsteps
mightinvolvelanders,andpossiblyrobotstomeltthroughtheiceusing
radioactiveheatgenerators.Eventuallyoneimaginessubmarinesdivingthougha
Europanocean.
ApartfromEnceladus,TitanisatargetofastrobiologyamongstSaturn’smoons.
Ahugescientificleapcouldbemadeifalakelander—asortofinterplanetary
boat—couldfloatonthelakesinthepolarregionsofTitanandfindoutwhich
substancesmakeuptheorganicliquids.Furthermore,aTitanorbitercoulddothe
kindofreconnaissancethatwoulddeterminethedepthofTitan’ssubsurface
oceanandstudyTitan’ssurface.
Onecertaintyforthefutureisthatexoplanetdiscoverieswillcontinuetospuran
interestinastrobiology.IanticipatethediscoveryofmanyEarth-likeplanets
insidethehabitablezoneofotherstars,deadplanetswithalmostpurecarbon
dioxideatmospheres,waterworldscoveredentirelyinglintingoceans,and
youngVenus-likeplanetssweatingofftheiroceansintospacefromrunaway
greenhouseeffects.
Whenastrobiologycametotheforeasadisciplineinthe1990s,some
questioneditsfutureandwonderedifitmightbeafadthatfades,perhaps
becauseofdisappointmentinnotquicklyfindingextraterrestriallifeorafailure
toanswerquestionsaboutlife’sorigin.However,thediscoveryofEarth-sized
exoplanetsinhabitablezoneswillensurethatthepossibilityoflifeelsewhere
becomesmorerelevantthanever.Astrobiologyisheretostay.
Furtherreading
Chapter1:Whatisastrobiology?
Muchlengthierintroductionstoastrobiologyarefoundinthefollowing
textbooks:
J.O.Bennett,G.S.Shostak.LifeintheUniverse.(SanFrancisco:Pearson
Addison-Wesley,2012).
D.A.Rotheryetal.AnIntroductiontoAstrobiology.(Cambridge:Cambridge
UniversityPress,2011).
K.W.Plaxco,M.Gross.Astrobiology:ABriefIntroduction.(Baltimore:Johns
HopkinsUniversityPress,2011).
J.I.Lunine.Astrobiology:AMultidisciplinaryApproach.(SanFranciso:Pearson
AddisonWesley,2005).
W.T.Sullivan,J.A.Baross(eds).PlanetsandLife:TheEmergingScienceof
Astrobiology.(Cambridge:CambridgeUniversityPress,2007).
Thedevelopmentofastrobiologyfromthe1950sonwardsisdescribedby:S.J.
Dick,J.E.Strick.TheLivingUniverse:NASAandtheDevelopmentof
Astrobiology.(NewBrunswick,NJ:RutgersUniversityPress,2004).
Anoldclassiconthenatureoflifeis:E.Schrödinger.WhatIsLife?(1944;
Cambridge:CambridgeUniversityPress,2012).
Chapter2:Fromstardusttoplanets,theabodesfor
life
AreadablediscussionofmodernBigBangtheoryisgivenby:C.Lineweaver,T.
Davis.2005.MisconceptionsabouttheBigBang.ScientificAmerican292:
36–45.
ApopularaccountofArthurHolmes’squesttofindtheageoftheEarthis:C.
Lewis.TheDatingGame:OneMan’sSearchfortheAgeoftheEarth.
(Cambridge:CambridgeUniversityPress,2012).
Chapter3:Originsoflifeandenvironment
Thescienceoftheoriginoflifeisdescribedby:R.M.Hazen.Genesis:The
ScientificQuestforLife’sOrigin.(Washington,DC:JosephHenryPress,
2005).
AluciddescriptionoftheearlyevolutionoflifeonEarthis:A.H.Knoll.Lifeon
aYoungPlanet:TheFirstThreeBillionYearsofEvolutiononEarth.
(Princeton:PrincetonUniversityPress,2003).
Chapter4:Fromslimetothesublime
TheEarth’sformation,evolution,andhabitabilityarecoveredin:C.H.
Langmuir,W.S.Broecker.HowtoBuildaHabitablePlanet:TheStoryof
EarthfromtheBigBangtoHumankind.(Princeton:PrincetonUniversity
Press,2012).
Chapter5:Life:agenome’swayofmakingmoreand
fittergenomes
Awidelyusedintroductorytextbookonmodernmicrobiologyis:M.T.Madigan
etal.BrockBiologyofMicroorganisms.(SanFrancisco:BenjaminCummings,
2012).
TheeffectsoflifeontheEarth’schemistryonagloballevelaredescribedinthe
followingtextbook:W.H.Schlesinger,E.S.Bernhardt.Biogeochemistry,
ThirdEdition:AnAnalysisofGlobalChange.(SanDiego:AcademicPress,
2013).
Chapter6:LifeintheSolarSystem
TheplanetsoftheSolarSystemandtheirhabitabilityaredescribedinthe
followingtextbook:J.J.Lissauer,I.dePater.FundamentalPlanetaryScience:
Physics,ChemistryandHabitability.(Cambridge:CambridgeUniversity
Press,2013).
Chapter7:Far-offworlds,distantsuns
Areadablebookthatdiscussesthesearchforhabitableexoplanetsis:J.F.
Kasting.HowtoFindaHabitablePlanet.(Princeton:PrincetonUniversity
Press,2010).
Chapter8:Controversiesandprospects
Thecontroversialbutengrossingbookthatarguesforthescarcityofcomplex
lifeis:P.D.Ward,D.Brownlee.RareEarth:WhyComplexLifeisUncommon
intheUniverse.(NewYork:Copernicus,2000).
Index
A
acetyleneonTitan106
adaptiveoptics113
aeonsonMars90
ageoftheEarth256
albedooftheEarth56
Aldebaran(star)18,20
ALH84001meteorite989
Alpha-CentauriB112
alterationminerals934
aluminium-26atoms223
AmesResearchCenter1
aminoacids76
ammonia(NH
3
)12,1079
Andromedagalaxy15
anti-biosignatures119
ApexChertrockformation,Australia42
archaea669,76,78,80
Archaeanaeon32,41,468,501
Aristotle3
asteroidimpacts2,24,25,30,612,96,126(seealsometeorites)
astrometry111
atmosphere845
onEarth28,324,4453,54,55,11819,125
onexoplanets59,11719
onMars84,85,8990,946,97
onPluto108
onTitan(moonofSaturn)1034
onTriton(moonofNeptune)107
onVenus84,858
atomscommontolife910
ATP(adenosinetriphosphate)35
B
bacteria669,756,78,98(seealsocyanobacteria)
survivalinextremeheat7980
bandedironformations50
Bernal,J.Desmond6
Betelgeuse19
BigBang14,1617
bioastronomy56
biomarkers423,54
biomass49,634,102,108
biomolecules701,73
chirality379
biosignatures11819
biospheres60,635,115,121
blackdwarfs19
blackholes20
bodystructures60
‘boringbillion’535
brainmass,proportional1223
Brasier,Martin42
Brock,Thomas7980
Brownlee,Don125
Budyko,Mikhail56
Buick,Roger40
C
Callisto(moonofJupiter)102
CambrianExplosion54,59,60
capcarbonates57
carbohydrates71,78
carbon,organic10,336,3940,42,5052,55,74
carbonassimilationexperiment96
carbon-basedlife910
carbondioxide11,32,34
intheEarth’satmosphere457,51,57
greenhouseeffect458,856,95,116
inthehabitablezone117,129
onMars84,85,89,946
onVenus84,85,86
carbonate–silicatecycle478
Cassini–Huygensmission1045
cells58,6570,78
circulation8
Ceres(largestasteroid)83,99
Charon(moonofPluto)108
chemicalelements65
chemicalweathering93
chemiosmosis35
chemoheterotrophs745
Chicxulub,Mexico612
chirality379
Chiron2930
chlorine589
chromosomes6970
Chyba,Christopher9
Cleland,Carol9
climate
onMars946
regulation478
CO
2
seecarbondioxide
Cocconi,Guiseppe120
coherentenergy67
continents,formation289
continuouslyhabitablezone(CHZ)115
convergentevolution122
CopernicanPrinciple1256
coronograph113
CosmicConnection(Sagan)13
CosmicMicrowaveBackground1617
cosmobiology6
Cosmotheoros(Huygens)4
Creataceous–Paleogenemassextinction612
Crick,Francis72
cryovolcanism1078
CuriosityRover8990
cyanobacteria423,4950,689,76
D
Darwin,Charles89,31,126
Dawnmission99
definitionofastrobiology12,5
Democritus3
densityofexoplanets11415
directdetectionofexoplanets11315
dissipativestructures8
DNA323,36,6970,713
DNAsequencing778
Dopplershift11112
Drake,Frank(DrakeEquation)12022
dropstones56
dustparticles10
dwarfs1921,106,11617
E
Earth84
age256
atmosphere28,324,4453,54,55,11819,125
basisoflife9,12,6370
earliestaeon2836
developmentofintelligentlife122,124
massextinctions602
originoflife12,3143
asa‘PaleBlueDot’118
position1415,267
SnowballEarthhypothesis558
uniqueness34,1257
Ediacarans54
Einstein,Albert113
elements
chemical65
commontolife910,89
isotopes223,256
non-metallic12
instars1718,19,21,117
enantiomers389
Enceladus(moonofSaturn)103,128
EncephalizationQuotient(EQ)1223
endosymbiosis69
energy69,87,106
infrared45
life-giving65
metabolic35,58,735
inphotons18
precursorforlife58
entropy68
eukaryotes59,6670,76,78,80
Europa(moonofJupiter)1002,1289
iceon23
EuropeanSpaceAgency114
evolution89,48,75
during‘boringbillion’535
chemical31
convergent122
diversityin60
exobiology5
exoplanets3,245
atmosphereon59,11719
detection11015
evidenceoflifeon11519
intelligentlifeon1204
expansionoftheuniverse1617
extinctions,mass602
extraterrestrialintelligence(SETI)1204
extraterrestriallifeevidenceof23,4
intelligent1204
likelihoodofcarbonbase910
likelihoodofsiliconbase1011
probability16
significance1213
theoriesabout34
extremophiles7981
F
faintyoungSunparadox445
Fermi,Enrico(FermiParadox)1234
fluorine589
fossils412,534,98
G
G-typestars1256
galacticfilaments16
galactichabitablezone(GHZ)15,11718,126
galaxies1516
birthof17
GalileanmoonsofJupiter99102
Galileo126
Galileospacecraft23,101,118
gametes6970
Ganymede(moonofJupiter)102
gaschromatographmassspectrometer(Vikinglander)97
gasexchangeexperiment(Vikinglander)967
gasgiants234
genetransfers77,79
genetics759
genomes8,367
giantimpacthypothesis26
giantplanets234
glaciations558
GreatOxidationEvent4853
greenhouseeffect458,856,116
onMars95
gulliesonMars91
H
habitablezone(HZ)11517
Hadeanaeon2836
Haldane,J.B.S.32
half-life22,25
Hawking,Stephen126
helium1718,20
Herrmann,Joachim5
Hertzsprung–Russell(H–R)diagram2021
Holmes,Arthur256
homochirality39
Hooker,Joseph31
hotJupiters245,110,112
Hubble,Edwin16
Huntress,Wes5
Huygens,Christiaan4,1045
hydrocarbonsonTitan106
hydrogen17,20,523,88
hydrogenbombs18
hydrogenperoxide90
hydrothermalvents345,42
I
ice
density1112
onEuropa(moonofJupiter)23
formation557
icegiants234
impacterosion96
impactswithEarth2830,334,612
indirectdetectionofexoplanets11013
infraredradiation456,87,93
innerplanets,wateron84
intelligentlifeonexoplanets1204
interferometry114
intraterrestriallife64
Io(moonofJupiter)100
iron50,51
iron-60
atoms23
isotopes223
radioactive256
Isua,Greenland3940
J
Jupiter65,85
formation24
moons23,99102,1289
orbit111
asprotectionforEarth1267
resonancewithSaturn30
K
Kstars116
Kant,Immanuel4,22
Kelvinscale20
Kepler,Johannes34
Keplermission11213,126
Kirschvink,Joe57
Klein,Harold‘Chuck’97
KuiperBeltobjects(KBOs)107,108
L
labelledreleaseexperiment97
Lafleur,Laurence45
LakeVostok,Antarctica801
Laplace,Simon-Pierre(MarquisdeLaplace)22
Laplaceresonance100
LateHeavyBombardment30,39
leadisotopes,age256
Lederberg,Joshua5
life
characteristics69
originonEarth3143
precursors589,65
light
bending113
cancelling114
lightyears15
lightning,creationoforganicmolecules32
Linnaeus,Carolus66
lipids71
liquidwater10,1112,65,826,89,906,99103,106,11516,119
LocalGroup15
Lowell,Percival4
M
Mdwarfs11617
MacGregor,Alexander32
magnesium-26atoms223
magneticfield56
onEuropa101
magnetite56,989
mainsequence201,44,11516
MarinermissionstoMars889
Mars846,8899
inthehabitablezone116
intelligentlife4
signsofancientlife2
MarsExplorationRovers94
MarsScienceLaboratoryseeCuriosityRover
massextinctions602
Mayor,Michel11112
mediumforbiochemicalprocesses11
Mercury845
messengerRNA(mRNA)73
metabolism35,58,735
metallicityofstars11718
meteorites25,334(seealsoasteroidimpacts)
age26
fromMars979
methane55,90
atmospheric467
onTitan1046
Methanopyruskandleri80
Metrodorus3
microbes645,127
extraterrestrial2
genetransfer77,79
survivalinextremeheat29,7980
microbialconjugation69
microbialmats401
microfossils412,98
microlensing113
MilkyWaygalaxy15,16
Miller,Stanley323
Mitchell,Peter35
molecularclocks789
molecules,organic423
Moon14
craters30
formation267,28
Morrison,Philip120
Murchisonmeteorite33
N
NASA1,2,5,889,99,101,112,114,118,126
naturalselection8,75
Nazcaplate,SouthPacific47
nebularhypothesis223,25
negativeentropy7
Neoproterozoicglaciations55,56,57
Neptune
distance15
formation24
moons1068
orbits30
neutrons19
Nicemodel30
non-metallicelements12
NorthPole,Australia401
nuclearfusion1718,19,21
nucleotides713
O
oceanicplates47
OfthePluralityofWorlds(Whewell)4
OortCloud126
Oparin,Alexander32
organiccarbon10,336,3940,42,502,55,74
organicmolecules326
originoflifeonEarth3143
OrionArmoftheMilkyWay15
outflowchannelsonMars913
oxygen32
onEuropa102
onexoplanets127
levels4853,54,55
precursorforlife589
oxygenationtime59
ozonelayer52
P
PAHs(polycyclicaromatichydrocarbons)98
‘PaleBlueDot’118
Paleaoproterozoicglaciations55,56,57
panspermia31
Patterson,Clair26
Pauling,Linus7,78
PCR(polymerasechainreaction)technology80
Pelagibacterubique645
Permian–Triassicmassextinction61
PhanerozoicAeon54
photonsintheSun18
photosynthesis4951
phylogeny779
Pikaia54
planetaryembryos24
planetarymigration25
planetarynebulae1819
planetesimals24
platetectonics48,127
Plato3
pluralism3
Pluto1089
primevallead26
primordialsoup32
prokaryotes678
proteinsynthesis77
proteins71,76
ProterozoicAeon55
protongradient35
ProximaCentauri15
Q
Queloz,Didier11112
R
racemicmixture38
radialvelocitymethod111
radioactiveisotopes256
radiogenicheat101
RareEarthHypothesis1257
recombination70
reddwarfs21
redgiants18
redsupergiants19
redoxtitration53
reductants52
ribosomes68
inRNA(rRNA)77
RNA367,39,713,77
rockyplanets21,23
runawaygreenhouseeffectonVenus867
runawaylimit878
S
Sagan,Carl13,14,127
samplereturnmissions1289
Saturn
formation24
moons82,1026,129
orbit111
resonancewithJupiter30
scablands93
scaleoftheuniverse1416
Schiaparelli,Giovanni4
Schneider,Eric8
Schopf,Bill42
Schrödinger,Erwin7,8
SecondLawofThermodynamics7
SecretService1
sedimentaryrocks3941,50
SETI(searchforextraterrestrialintelligence)1204
sexualreproductionearly54
ofeukaryotes6970
Shapley,Harlow115
siliconbasedlife1011
Simpson,GeorgeGaylord5
sizeofMars96
SnowballEarthhypothesis558
socialanimals123
soilonMars967
solarflux85,87
SolarSystem15
age256
formation225
planetaryorbits301
possibleabodesoflife83109,1289
SouthAfrica,microfossils42
species756
spectraofexoplanets119
spectralclass21
SPONCHelements12
stablemediumforbiochemicalprocesses11
stars,lifecycle1821
StrelleyPoolFormation,Australia42
stromatolites401,50
Struve,Otto5
subduction47
subsurfacebiosphere64
sulphatesonMars94
sulphur512
sulphurdioxideonMars95
Sun1415,445
elementsin1718
lifecycle1819,21
Super-Earths110
supernovae1920,21,23
surfacestability65
symmetryseechirality
synchronousrotation11617
T
taxonomiclevels66
technosignatures1204
telescopes,space-based11314
Thales3
Theia,impactwithEarth26
TheoryofGeneralRelativity113
thermodynamics68
thermophiles2930,7981
Thermusaquaticus80
tidallocking11617
Tikov,Gavriil5
tillites56
tilt127
ofMars95
Titan(moonofSaturn)82,1036,129
transcription73
transitmethod11213,119
transittimingvariations114
TransitingExoplanetSurveySatellite(TESS)113
transmittingcivilizations1204
treeoflife76,77
Triton(moonofNeptune)1068
TumbianaFormation,Australia50
U
universe
expansion1617
size1416
uraniumisotopes,age25
Uranus
formation24
orbits30
Urey,Harold323
V
valleynetworksonMars912
Venus848
Vikinglanders9,89,967
Virgosupercluster15
viruses70
volcanoes33,57
onIo100
causeofmassextinction61
Voyager1spacecraft118
W
Ward,Peter125
water,liquid10,1112,65,826,89,906,99103,106,11516,119
WaterbeltEarth578
Watson,James72
weirdlife82,106
Whewell,William4,115
whitedwarfs19
wobblesinstellarorbits111
Woese,Carl767
Z
zircons29
Zuckerkandl,Emile78
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GALAXIES
AVeryShortIntroduction
JohnGribbin
GalaxiesarethebuildingblocksoftheUniverse:standinglikeislandsinspace,
eachismadeupofmanyhundredsofmillionsofstarsinwhichthechemical
elementsaremade,aroundwhichplanetsform,andwhereonatleastoneof
thoseplanetsintelligentlifehasemerged.InthisVeryShortIntroduction,
renownedsciencewriterJohnGribbindescribestheextraordinarythingsthat
astronomersarelearningaboutgalaxies,andexplainshowthiscanshedlighton
theoriginsandstructureoftheUniverse.
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PLANETS
AVeryShortIntroduction
DavidA.Rothery
ThisVeryShortIntroductionlooksdeepintospaceanddescribestheworldsthat
makeupourSolarSystem:terrestrialplanets,giantplanets,dwarfplanetsand
variousotherobjectssuchassatellites(moons),asteroidsandTrans-Neptunian
objects.Itconsidershowourknowledgehasadvancedoverthecenturies,and
howithasexpandedatagrowingrateinrecentyears.DavidA.Rotherygives
anoverviewoftheorigin,nature,andevolutionofourSolarSystem,including
thecontroversialissuesofwhatqualifiesasaplanet,andwhatconditionsare
requiredforaplanetarybodytobehabitablebylife.Helooksatrockyplanets
andtheMoon,giantplanetsandtheirsatellites,andhowthesurfaceshavebeen
sculptedbygeology,weather,andimpacts.
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STARS
AVERYSHORTINTRODUCTION
AndrewKing
Everyatomofourbodieshasbeenpartofastar.Ourveryownstar,theSun,is
crucialtothedevelopmentandsustainabilityoflifeonEarth.ThisVeryShort
Introductionpresentsamodern,authoritativeexaminationofhowstarslive,
producingallthechemicalelementsbeyondhelium,andhowtheydie,
sometimesspectacularly,toendasremnantssuchasblackholes.
AndrewKingshowshowunderstandingthestarsiskeytounderstandingthe
galaxiestheyinhabit,andthusthehistoryofourentireUniverse,aswellasthe
existenceofplanetslikeourown.Kingpresentsafascinatingexplorationofthe
scienceofstars,fromthemechanismsthatallowstarstoformandtheprocesses
thatallowthemtoshine,aswellastheresultsoftheirinevitabledeath.
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